Category Archives: Prime numbers

Altars of Mathness

by in Album Art, Arithmetic, Base Behavior, Fibonacci Sequence, Fractals, Integer Sequences, Mathematics, Polygonal numbers, Powers, Prime numbers, Triangular Numbers and tagged , , , , , , , , , , , | Leave a comment

What could be duller than digits? They just sit there on the page or screen, mindlessly marking mathematics:

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100…

But perhaps they become more interesting as images. Let’s display the final digit of the integers, or counting numbers, on a graph. Running left-right and up-down, the graph represents the final or rightmost digit of 1, 2, 3, … 10, 11, 12, 13, … 100, 101, 102, 103, …, 1000, 1001, 1002, 1003, …:

Rightmost single digit of the integers (click for larger)

No, that’s still dull: the graph just generates endlessly repeating triangles. After all, the final digits fall into a cycle: 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3… So do the final two digits: 1, 2, 3, 4, 5, […] 94, 95, 96, 97, 98, 99, 00, 01, 02, 03… Here they are as a graph:

Rightmost two digits of the integers

Now the triangles look like waves sweeping to shore. That’s a bit more interesting, but not much. So let’s try something different. The trailing digits of the integers generate triangles, so let’s see what the triangular numbers generate. The triangular numbers — 0, 1, 3, 6, 10, 15, 21… — are very simple to form. You just sum the integers: 1, 3 = 1 + 2, 6 = 1 + 2 + 3, 10 = 1 + 2 + 3 + 4, 15 = 1 + 2 + 3 + 4 + 5, 21 = 1 + 2 + 3 + 4 + 5 + 6, 28 = 1 + 2 + 3 + 4 + 5 + 6 + 7, 36 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8, 45 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9, 55 = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10… Here are the final digits of the triangulars — 1, 3, 6, 0, 5, 1, 8, 6… — as a graph:

Final digit of triangular numbers in base 10 (click for larger)

Now something interesting has appeared. The final digits form a repeated palindromic pattern (counting 0 as the zero-th triangular number):

0, 1, 3, 6, 0, 5, 1, 8, 6, 5, 5, 6, 8, 1, 5, 0, 6, 3, 1, 0, 0, 1, 3, 6, 0, 5, 1, 8, 6, 5, 5, 6, 8, 1, 5, 0, 6, 3, 1, 0, 0, 1, 3, 6, 0, 5, 1, 8, 6, 5, 5, 6, 8, 1, 5, 0, 6, 3, 1, 0, 0, 1, 3, 6, 0, 5, 1, 8, 6, 5, 5, 6, 8, 1, 5, 0, 6, 3, 1, 0, …

An Altar of Mathness created by the final digit of triangular numbers in base 10

And those palindromic digits create symmetric shapes that remind me of little altars — let’s call them “altars of mathness” in tribute to Morbid Angel’s genre-defining album Altars of Madness (1989). And what about the final two digits of the triangular numbers? Here’s the graph (adjusted so that 99 fits into the same space as 9):

Final two digits of triangulars in b10

Final two triangular digits in b10 (horizontal scale compressed)

The final two digits form palindromes too. And this time we don’t get just triangles, but curves too. But that’s in base 10. What happens with the trailing triangular digits in other bases? Well, here’s the final triangular digit creating more altars of mathness in different bases (note that the altars are more elaborate in even bases):

Final triangular digit in base 4

Final triangular digit in b5

Final triangular digit in b6

Final triangular digit in b7

Final triangular digit in b8

Final triangular digit in b9

Final triangular digit in b14

And here’s the graph for the final triangular digit in base 100:

Final triangular digit in b100

The graph for final single digit in b100 should look familiar, because it’s identical to the graph for final double triangular digits in b10:

Final two digits of triangulars in b10

That’s because two digits in b10 are equivalent in one digit in b100, four digits in b10 are equivalent to two digits in b100, and so on. But b100 can’t capture three digits in b10 (the graph is again adjusted so that 999 fits into the same space as 9 and 99 above):

Final three triangular digits in b10

If you compress the x-axis for that graph, you can see how long the symmetries are:

Final three triangular digits in b10 (x-axis / 2)

Final three triangular digits in b10 (x-axis / 4)

The final four digits of the triangulars in b10 create even longer symmetries:

Final quadruple triangular digits in b10

Final quadruple triangular digits in b10 (x-axis / 2)

Final quadruple triangular digits in b10 (x-axis / 8)

Note how, as the length of the final digits rises, you need to compress the x-axis more and more to see the symmetries. But integer sequences obviously don’t end with the counting numbers and triangulars. What about squares and powers of n? What about primes and Fibonacci numbers? Here’s the final two digits of the squares — 1, 4, 9, 16, 25, 49, 64, 81, 100, 121, 144, 169… — in b10:

Final two digits of the squares in b10

It’s reminiscent of the triangular numbers (so are the final-digit graphs for other polygonal numbers). So what about the powers of 2? That’s 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024… Here’s the graph for final single digits of 2^p in b10:

Final single digits of 2^p in b10

This time there’s repetition, but not symmetry. Here’s the graph for final double digits, or 2-digits, of 2^p in b10:

Final 2-dig of 2^p in b10

Now the graph looks a little like a range of eroded mountains. Now try dig-4, the final four digits of 2^p in b10:

Final 4-dig of 2^p in b10

The patterns are similar to those of dig-2 and don’t need compressing in the x-axis. This similarity and lack of need for compression are true of any number of final digits in 2^p. The final 10 digits look like this:

Final 10-dig of 2^p in b10

And the final 20 and 30 digits like this:

Final 20-dig of 2^p in b10

Final 30-dig of 2^p in b10

Powers don’t behave like polygonals: the finals are fractals. That is, the final digits create similar patterns at all scales: 1-dig, 2-dig, 10-dig, 100-dig, 1000-dig and so on. That’s true in other bases:

Final 5-dig of 3^p in b2

But a glimpse of b2 is all you’re going to get of other bases. There are other fish to fry — Fibonacci fish. The Fibonacci sequence, whose terms are equal to the sum of the previous two numbers (after seeding with “1, 1”), starts like this: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811… And what about the graphs for final fib-digits? As you’ll see, final Fib-digits are fractal too. Indeed, Fibonacci final-graphs look like 2-power final-graphs (in a way, Fibonacci numbers are powers of φ = 1.6180339887498948482…). The patterns are similar at all scales. And they remind me of the skyline of a ruined city in an Oriental tale, with collapsed domes and crumbling minarets:

Final 1-dig of Fibonacci numbers in b10

Final 2-fibdig in b10

Final 3-fibdig in b10

Final 4-fibdig in b10

Final 5-fibdig in b10

Final 10-fibdig in b10

Final 15-fibdig in b10

Final 20-fibdig in b10

Final 25-fibdig in b10

So final fibdigs are fractal. But final prime digits aren’t:

Final 1-digit of primes in b10

Final 1-digit of primes in b5

Final 2-digit of primes in b10

Primes aren’t final-digitally fractal like Fibonaccis and powers of 2. But there’s occasional symmetry in the prime fin-digs. I’ve marked some palindromic patterns in red and green:

Palindromic patterns in final 1-digits of the primes in b10 (click for larger)

The palindromic patterns, or pal-pats, in the primes look like the altars of mathness in the triangulars. They’re created by digital palindromes like these:

19, 23, 29 (c=3)
347, 349, 353, 359, 367 (c=5)
937, 941, 947, 953, 967, 971, 977 (c=7)
1951, 1973, 1979, 1987, 1993, 1997, 1999, 2003, 2011 (c=9)
26423, 26431, 26437, 26449, 26459, 26479, 26489, 26497, 26501, 26513 (c=10)

Here are the first few pal-pats in the primes (note that 157, 163, 167 and 163, 167, 173 overlap):

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607…

And are there palindromes among the final 2-digits, 3-digits and higher n-digits of the primes in different bases? Yes, you can easily find some. But I haven’t put them on a graph yet:

Base 10 (2-dig)

58789, 58831, 58889 (c=3)
286873, 286927, 286973 (c=3)
360649, 360653, 360749 (c=3)
404851, 404941, 404951 (c=3)
590437, 590489, 590537 (c=3)
623071, 623107, 623171 (c=3)
651517, 651587, 651617 (c=3)

Base 6 (2-dig)

300335, 300401, 300441, 300501, 300535 (c=5) (23459 to 23531 in base 10)
1030255, 1030331, 1030351, 1030431, 1030455 (c=5) (50651 to 50723 in b10)
1140451, 1140501, 1140521, 1141001, 1141051 (c=5) (59791 to 59863 in b10)
1402451, 1402545, 1403031, 1403045, 1403051 (c=5) (78367 to 78439 in b10)
1435431, 1435451, 1435505, 1435551, 1440031 (c=5) (82891 to 82963)
2400505, 2401001, 2401015, 2401101, 2401105 (c=5) (124601 to 124673)
2442235, 2442311, 2442351, 2442411, 2442435 (c=5) (130127 to 130199)
2444215, 2444225, 2444311, 2444325, 2444415 (c=5) (130547 to 130619)
2533105, 2533121, 2533215, 2533221, 2533305 (c=5) (136769 to 136841)

Base 4 (3-dig)

20013013, 20013133, 20020013 (c=3) (33223 to 33287 in base 10)
21031111, 21031303, 21032111 (c=3) (37717 to 37781)
22310011, 22310333, 22311011 (c=3) (44293 to 44357)
33030121, 33031001, 33031121 (c=3) (62233 to 62297)
102031333, 102032131, 102032333 (c=3) (74623 to 74687)
110013121, 110013311, 110020121 (c=3) (82393 to 82457)

Base 3 (3-dig)

112121020012, 112121021211, 112121022021, 112121100211, 112121101012 (c=5) (287393 to 287501 in base 10)
202002212002, 202002212101, 202002220001, 202002221101, 202010000002 (c=5) (395741 to 395849)
1001012111212, 1001012112202, 1001012121022, 1001012121202, 1001012122212 (c=5) (555143 to 555251)
1010112112012, 1010112112201, 1010112120222, 1010112121201, 1010112200012 (c=5) (601079 to 601187)
1011202211211, 1011202212212, 1011202220022, 1011202221212, 1011202222211 (c=5) (625369 to 625477)

Base 2 (5-dig)

101110001111101, 101110010000111, 101110010001001, 101110010100111, 101110010111101 (c=5) (23677 to 23741 in base 10)
10000001000111101, 10000001001011001, 10000001001110001, 10000001001111001, 10000001001111101 (c=5) (66109 to 66173)
10111100110011011, 10111100110011111, 10111100110111001, 10111100110111111, 10111100111011011 (c=5) (96667 to 96731)
11010000111110001, 11010001000001101, 11010001000011001, 11010001000101101, 11010001000110001 (c=5) (106993 to 107057)

And I conjecture that you’ll get palindromes for any number of final digits in all bases. And can these palindromes be of arbitrary length? Again, I conjecture so. There are infinitely many primes and very rare patterns can occur infinitely often in an infinite set of numbers.

Post-Performative Post-Scriptum

Here’s Dan Seagrave’s classic cover for Morbid Angel’s Altars of Madness (1989):

• Morbid Angel — official website
• Dan Seagrave — official website

Elsewhere Other-Accessible…

• Formulas Focal to the Flesh — a pre-previous post paronomasizing the title of a Morbid-Angel album…

Sieve and Let Spi’

by in Arithmetic, Mathematics, Prime numbers, Ulam Spirals and tagged , , , , , , , , | Leave a comment

What is VDSP? Inter alia, it’s the complicated consonant-cluster you get when you carefully pronounce the phrase “sieved spiral”. And here is a sieved spiral:

An Ulam Spiral of primes represented on a square grid

The pattern above is called an Ulam spiral (OO-lam) after its inventor, the Polish-Jewish mathematician Stanisław Ulam (1909-84). The white squares represent the prime numbers as you spiral counter-clockwise on a square grid — the little boot or reversed-L in the middle is the only time that filled squares are in direct contact, because it includes square #2, the only even prime. #2 is the heel of the boot, with #3 as the shaft and #11 as the toe.

But the Ulam spiral could also be called a sieved spiral, because you can build it by using the Sieve of Erastosthenes, whose invention is attributed to the Greek scholar Eratosthenes of Cyrene (c. 276–c. 194 BC). Create a list of whole numbers skipping 1. Then choose the first number on the list, which is 2. Cross out every higher number that’s divisible by 2. Then choose the next number that isn’t crossed out. It’ll be 3. Cross out every higher number that’s divisible by 3. Then choose the next available number, 5, and cross out all higher numbers divisible by 5. When you’ve crossed out everything you can, you’ll be left with just prime numbers. Here’s an animation of the Sieve from Wikipedia:

Animated Sieve of Erastosthenes from Wikipedia

Now we can sieve-and-let-spi’, as it were. First create a square grid with white squares. Choose the square in the middle as #1 and fill it it. Then choose white square to the right of #1 and call it #2. Then spiral outwards counter-clockwise filling with black all squares whose count is divisible by 2. Then do that for squares #3, #5, #7, #11 and so on. In the end, the only white squares on the grid will be the primes. And you’ll have a sieved Ulam spiral:

Sieving a spiral — creating the Ulam spiral using the Sieve of Eratosthenes

You can also sieve and let spi’ in reverse, blacking the squares using primes from higher to lowest. With this method, the sieved spiral looks like this:

Sieving a spiral — creating the Ulam spiral using the Sieve of Eratosthenes (higher primes first)

Graph durch Euler

by in Arithmetic, Mathematics, Prime numbers and tagged , , , , , , , , , , , , , | Leave a comment

This is the famous Ulam spiral, in which prime numbers are represented on filled squares on a square spiral:

The Ulam spiral

I like the way the spiral sits between chaos and calm. It’s not wholly random and it’s not wholly regular — it’s betwixt and between. You get a similar chaos-and-calm vibe from a graph for a function called Euler phi. And primes are at work there too. Here’s the graph from Wikipedia:

Graph of eulerphi(n) = φ(n) (see Euler’s totient function)

But what is the Euler phi function? For any integer n, eulerphi(n) gives you the count of numbers < n that are relatively prime to n. That is, the count of numbers < n that have no common factors with n other than one. You can see how eulerphi(n) works by considering whether you can simplify the fraction a/b, where a = 1..n-1 and b = n:

φ(6) = 2
1/6 (1)
2/6 → 1/3
3/6 → 1/2
4/6 → 2/3
5/6, ∴ φ(6) = 2

φ(7) = 6
1/7 (1)
2/7 (2)
3/7 (3)
4/7 (4)
5/7 (5)
6/7, ∴ φ(7) = 6

φ(12) = 4
1/12 (1)
2/12 → 1/6
3/12 → 1/4
4/12 → 1/3
5/12 (2)
6/12 → 1/2
7/12 (3)
8/12 → 2/3
9/12 → 3/4
10/12 → 5/6
11/12, ∴ φ(12) = 4

φ(13) = 12
1/13 (1)
2/13 (2)
3/13 (3)
4/13 (4)
5/13 (5)
6/13 (6)
7/13 (7)
8/13 (8)
9/13 (9)
10/13 (10)
11/13 (11)
12/13, ∴ φ(13) = 12

As you can see, eulerphi(n) = n-1 for primes. Now you know what the top line of the Eulerphi graph is. It’s the primes. Here’s a bigger version of the graph:

Graph of eulerphi(n) = φ(n)

Unlike the Ulam spiral, however, the Eulerphi graph is cramped. But it’s easy to stretch it. You can represent φ(n) as a fraction between 0 and 1 like this: phifrac(n) = φ(n) / (n-1). Using phifrac(n), you can create Eulerphi bands, like this:

Eulerphi band, n <= 1781

Eulerphi band, n <= 3561

Eulerphi band, n <= 7121

Eulerphi band, n <= 14241

Or you can create Eulerphi discs, like this:

Eulerphi disc, n <= 1601

Eulerphi disc, n <= 3201

Eulerphi disc, n <= 6401

Eulerphi disc, n <= 12802

Eulerphi disc, n <= 25602

But what is the bottom line of the Eulerphi bands and inner ring of the Eulerphi discs, where φ(n) is smallest relative to n? Well, the top line or outer ring is the primes and the bottom line or inner ring is the primorials (and their multiples). The function primorial(n) is the multiple of the first n primes:

primorial(1) = 2
primorial(2) = 2*3 = 6
primorial(3) = 2*3*5 = 30
primorial(4) = 2*3*5*7 = 210
primorial(5) = 2*3*5*7*11 = 2310
primorial(6) = 2*3*5*7*11*13 = 30030
primorial(7) = 2*3*5*7*11*13*17 = 510510
primorial(8) = 2*3*5*7*11*13*17*19 = 9699690
primorial(9) = 2*3*5*7*11*13*17*19*23 = 223092870
primorial(10) = 2*3*5*7*11*13*17*19*23*29 = 6469693230

Here are the numbers returning record lows for φfrac(n) = φ(n) / (n-1):

φ(4) = 2 (2/3 = 0.666…)
4 = 2^2
φ(6) = 2 (2/5 = 0.4)
6 = 2.3
φ(12) = 4 (4/11 = 0.363636…)
12 = 2^2.3
[…]
φ(30) = 8 (8/29 = 0.275862…)
30 = 2.3.5
φ(60) = 16 (16/59 = 0.27118…)
60 = 2^2.3.5
[…]
φ(210) = 48 (48/209 = 0.229665…)
210 = 2.3.5.7
φ(420) = 96 (96/419 = 0.2291169…)
420 = 2^2.3.5.7
φ(630) = 144 (144/629 = 0.228934…)
630 = 2.3^2.5.7
[…]
φ(2310) = 480 (480/2309 = 0.2078822…)
2310 = 2.3.5.7.11
φ(4620) = 960 (960/4619 = 0.20783719…)
4620 = 2^2.3.5.7.11
[…]
30030 = 2.3.5.7.11.13
φ(60060) = 11520 (11520/60059 = 0.191811385…)
60060 = 2^2.3.5.7.11.13
φ(90090) = 17280 (17280/90089 = 0.1918103209…)
90090 = 2.3^2.5.7.11.13
[…]
φ(510510) = 92160 (92160/510509 = 0.18052571061…)
510510 = 2.3.5.7.11.13.17
φ(1021020) = 184320 (184320/1021019 = 0.18052553…)
1021020 = 2^2.3.5.7.11.13.17
φ(1531530) = 276480 (276480/1531529 = 0.180525474868579…)
1531530 = 2.3^2.5.7.11.13.17
φ(2042040) = 368640 (368640/2042039 = 0.18052544540040616…)
2042040 = 2^3.3.5.7.11.13.17

Primal Polynomial

by in Arithmetic, Mathematics, Prime numbers and tagged , , , | Leave a comment

n² + n + 17 is one of the best-known polynomial formulas for primes. Its values for n = 0 to 15 are all prime, starting with 17 and ending with 257. — David Wells in The Penguin Dictionary of Curious and Interesting Numbers (1986), entry for “17”

• 17, 19, 23, 29, 37, 47, 59, 73, 89, 107, 127, 149, 173, 199, 227, 257

Primal Pellicles

by in Arithmetic, Base Behavior, Integer Sequences, Mathematics, Prime numbers and tagged , , , , , , , , | Leave a comment

Numbers have thin skins. And they’re easily replaced. Take 71624133. Here it is permuting its pellicles:

71624133 in base 10 = 100010001001110010111000101 in base 2 = 11222202212211200 in b3 = 10101032113011 in b4 = 121313433013 in b5 = 11035053113 in b6 = 1526536500 in b7 = 421162705 in b8 = 158685750 in b9 = 374802A9 in b11 = 1BBA1199 in b12 = 11AB9B59 in b13 = 9726137 in b14 = 644BE73 in b15 = F3855B7 in b16

But if digits are the skin of 71624133, what are its bones? Well, you could say the skeleton of a number, something that doesn’t change from base to base, is its prime factorization:

71624133 = 32 × 72 × 162413

But the primes themselves are numbers, so they’re wearing pellicles too. And it turns out that, in base 10, the pellicles of the prime factors of 71624133 match the pellicle of 71624133 itself:

71624133 = 32.72.162413

Here’s a list of primal pellicles in base 10:

735 = 3.5.72
3792 = 24.3.79
1341275 = 52.13.4127
13115375 = 53.7.13.1153
22940075 = 52.229.4007
29373375 = 3.53.29.37.73
71624133 = 32.72.162413
311997175 = 52.7.172.31.199
319953792 = 27.3.53.79.199
1019127375 = 32.53.7.127.1019
1147983375 = 3.53.7.11.83.479
1734009275 = 52.173.400927
5581625072 = 24.5581.62507
7350032375 = 53.7.23.73.5003
17370159615 = 34.5.17.59.61.701
33061224492 = 22.33.306122449
103375535837 = 72.37.103.553583
171167303912 = 23.11.172.6730391
319383665913 = 3.133.19.383.6659
533671737975 = 34.52.17.53.367.797
2118067737975 = 32.52.7.79.211.80677
3111368374257 = 3.112.132.683.74257
3216177757191 = 3.73.191.757.21617
3740437158475 = 52.37.4043715847
3977292332775 = 3.52.292.233.277.977
4417149692375 = 53.7.23.4969.44171
7459655393232 = 24.32.72.23.45965539
7699132721175 = 3.52.72.27211.76991
7973529228735 = 3.5.7.972.2287.3529
10771673522535 = 34.5.67.71.107.52253

You can find them at the Online Encyclopedia of Integer Sequences under A121342, “Composite numbers that are a concatenation of their distinct prime divisors in some order.” But what about pairs of primal pellicles, that is, pairs of numbers where the prime factors of each form the pellicle of the other?

35 = 5.7 ↔ 75 = 3.52
1275 = 3.52.17 ↔ 3175 = 52.127
131715 = 32.5.2927 ↔ 329275 = 52.13171
3199767 = 3.359.2971 ↔ 35932971 = 3.19.67.972
14931092 = 22.11.61.5563 ↔ 116155632 = 24.3.109.1492

And here are a few primal pellicles I’ve found in other bases:

Primal Pellicles in Base 2

1111011011110 = 10.1110.110110111 in b2 = 7902 = 2.32.439 in b10
1110001100110111 = 1110.10111.100011001 in b2 = 58167 = 32.23.281 in b10
1111011011011110 = 10.1110.110110110111 in b2 = 63198 = 2.32.3511 in b10
11101001100001101 = 1110.101.101001100001 in b2 = 119565 = 32.5.2657 in b10
1111011011011011110 = 10.1110.110110110110111 in b2 = 505566 = 2.32.28087 in b10
1111011111101111011 = 1110.1011.10111.11011111 in b2 = 507771 = 32.11.23.223 in b10

Primal Pellicles in Base 3

121022 = 210.12.102 in b3 = 440 = 23.5.11 in b10
212212 = 22.21.212 in b3 = 644 = 22.7.23 in b10
20110112 = 210.201.1011 in b3 = 4712 = 23.19.31 in b10
21110110 = 10.212.1101 in b3 = 5439 = 3.72.37 in b10
121111101 = 122.111.1101 in b3 = 12025 = 52.13.37 in b10
222112121 = 22.21.221121 in b3 = 19348 = 22.7.691 in b10
2202122021 = 22.2021.22021 in b3 = 54412 = 22.61.223 in b10
120212201221 = 2.122.21.201.1202 in b3 = 312550 = 2.52.7.19.47 in b10

Primal Pellicles in Base 7

2525 = 2.52.25 in b7 = 950 = 2.52.19 in b10
3210 = 2.34.10 in b7 = 1134 = 2.34.7 in b10
5252 = 2.52.52 in b7 = 1850 = 2.52.37 in b10
332616 = 33.16.326 in b7 = 58617 = 33.13.167 in b10
336045 = 32.5.3604 in b7 = 59715 = 32.5.1327 in b10
2251635 = 22.3.5.16.252 in b7 = 281580 = 22.3.5.13.192 in b10

Primal Pellicles in Base 11

253 = 22.3.52 in b11 = 300 = 22.3.52 in b10
732 = 2.32.72 in b11 = 882 = 2.32.72 in b10
2123 = 23.33.12 in b11 = 2808 = 23.33.13 in b10
3432 = 25.3.43 in b11 = 4512 = 25.3.47 in b10
3710 = 32.72.10 in b11 = 4851 = 32.72.11 in b10
72252 = 23.72.225 in b11 = 105448 = 23.72.269 in b10

Primal Pellicles in Base 15

275 = 24.5.7 in b15 = 560 = 24.5.7 in b10
2D5 = 2.52.D in b15 = 650 = 2.52.13 in b10
2CD5 = 2.52.CD in b15 = 9650 = 2.52.193 in b10
7BE3 = 3.72.BE in b15 = 26313 = 3.72.179 in b10
21285 = 24.52.128 in b15 = 105200 = 24.52.263 in b10

The Fatal Factory

by in Arithmetic, Mathematics, Prime numbers, Puzzles, Squares and tagged , , , , , , , , , , , , , , , | Leave a comment

I can’t remember where I came across this clever little puzzle and what precise form it took, but here’s my version of it:

A famously eccentric inventor and recreational mathematician has invited you to tour the factory where his company manufactures locks, keys, safes, cash-boxes and so on. At the end of the tour he brings you to a conference room, pours you a glass of wine, and invites you to test your wits against a puzzle. He points out that a hundred numbered boxes have been set out on two long tables in the room. You sip your wine as you listen to him explain that each box is locked and contains a slip of paper bearing a number between 0 and 9. If you accept the challenge, the inventor will order a hundred workers to walk in turn past the boxes, using a master-key to unlock or lock the boxes like this:

The first worker will use the key on every box (boxes #1,2,3…), the second worker will use the key on every second box (boxes #2,4,6…), the third worker the key on every third box (boxes #3,6,9…), and so on.

Now, you can’t tell by simply looking at a box whether it’s unlocked or not, but it’s obvious that the first box will be unlocked when all that is over. Box #1 is originally locked and the master-key will be used on it just once. But how many other boxes will be unlocked? If you can choose nothing but the unlocked boxes, you get to keep the contents. Otherwise you get nothing. That is, if you choose one or more locked boxes, you get nothing.

And what good are the contents of the unlocked boxes? Well, if you take the numbered slips of paper they contain in order, they will give you the combination of a locked safe the inventor now points out in the wall behind you. The safe contains the antidote for the deadly but slow-acting poison he secretly slipped into the wine you have been sipping as you listened to him explain the details of the puzzle. So you have to choose all and only the unlocked boxes to save your life. Can you do it?

Solution

I’m sure there’s a simpler explanation of which boxes will be unlocked, but here’s my complicated one:

Whether box #n is locked or unlocked in the end depends on how many divisors the number n has. If it has an even number of divisors, it will be locked; if it has an odd number of divisors, it will be unlocked. Take box #12. The number 12 has six divisors: 1, 2, 3, 4, 6 and 12. So workers #1, #3 and #6 will unlock it with the master-key, but workers #2, #4 and #12 will lock it again. Worker #12 will be the final worker to use the master-key on the box, so it will be locked.

Now take box #16. The number #16 has five divisors: 1, 2, 4, 8 and 16. So workers #1, #4 and #16 will unlock the box with the master-key, while workers #2 and #8 will lock it. Worker #16 will be the final worker to use the master-key on the box, so it will be unlocked.

In other words, the puzzle reduces to this: Which numbers from 1 to 100 have an odd number of divisors? To work out the number of divisors n has, you add 1 to the exponent of each of its prime factors and multiply the results. For example, 24 has eight divisors thus:

• 24 = 2^3 * 3^1 → (3+1) * (1+1) = 4 * 2 = 8, so 24 has eight divisors: 1, 2, 3, 4, 6, 8, 12, 24

But 36 has nine divisors thus:

• 36 = 2^2 * 3^2 → (2+1) * (2+1) = 3 * 3 = 9, so 36 has nine divisors: 1, 2, 3, 4, 6, 9, 12, 18, 36

36 demonstrates that a number has to have only even exponents on its prime factors to have an odd number of divisors (the only number without prime factors is 1, which has one divisor, namely itself). Numbers with only even exponents on their prime factors are square numbers:

• 4 = 2^2 → (2+1) = 3, so 4 has three divisors: 1, 2, 4
• 9 = 3^2 → (2+1) = 3, so 9 has three divisors: 1, 3, 9
• 16 = 2^4 → (4+1) = 5, so 16 has five divisors: 1, 2, 4, 8, 16
• 25 = 5^2 → (2+1) = 3, so 25 has divisors: 1, 5, 25
• 36 = 2^2 * 3^2 → (2+1) * (2+1) = 3 * 3 = 9, so 36 has nine divisors: 1, 2, 3, 4, 6, 9, 12, 18, 36
• 49 = 7^2 → (2+1) = 3, so 49 has three divisors: 1, 7, 49
• 64 = 2^6 → (6+1) = 7, so 64 has seven divisors: 1, 2, 4, 8, 16, 32, 64
• 81 = 3^4 → (4+1) = 5, so 81 has five divisors: 1, 3, 9, 27, 81
• 100 = 2^2 * 5^2 → (2+1) * (2+1) = 3 * 3 = 9, so 100 has nine divisors: 1, 2, 4, 5, 10, 20, 25, 50, 100

So if you choose boxes #1, #4, #9, #16, #25, #36, #49, #64, #81 and #100, you’ll get the combination for the safe and save your life.

Appendix

Here’s the full description of what happens to the boxes:

• box #1 is unlocked by worker #1 and locked by no-one, therefore it’s unlocked
• box #2 is unlocked by worker #1 and locked by worker #2, therefore it’s locked
• box #3 is unlocked by worker #1 and locked by worker #3, therefore it’s locked
• box #4 is unlocked by workers #1 and #4, and locked by worker #2, therefore it’s unlocked
• box #5 is unlocked by worker #1 and locked by worker #5, therefore it’s locked
• box #6 is unlocked by workers #1 and #3, and locked by workers #2 and #6, therefore it’s locked
• box #7 is unlocked by worker #1 and locked by worker #7, therefore it’s locked
• box #8 is unlocked by workers #1 and #4, and locked by workers #2 and #8, therefore it’s locked
• box #9 is unlocked by workers #1 and #9, and locked by worker #3, therefore it’s unlocked
• box #10 is unlocked by workers #1 and #5, and locked by workers #2 and #10, therefore it’s locked
• box #11 is unlocked by worker #1 and locked by worker #11, therefore it’s locked
• box #12 is unlocked by workers #1, #3 and #6, and locked by workers #2, #4 and #12, therefore it’s locked
• box #13 is unlocked by worker #1 and locked by worker #13, therefore it’s locked
• box #14 is unlocked by workers #1 and #7, and locked by workers #2 and #14, therefore it’s locked
• box #15 is unlocked by workers #1 and #5, and locked by workers #3 and #15, therefore it’s locked
• box #16 is unlocked by workers #1, #4 and #16, and locked by workers #2 and #8, therefore it’s unlocked
• box #17 is unlocked by worker #1 and locked by worker #17, therefore it’s locked
• box #18 is unlocked by workers #1, #3 and #9, and locked by workers #2, #6 and #18, therefore it’s locked
• box #19 is unlocked by worker #1 and locked by worker #19, therefore it’s locked
• box #20 is unlocked by workers #1, #4 and #10, and locked by workers #2, #5 and #20, therefore it’s locked
• box #21 is unlocked by workers #1 and #7, and locked by workers #3 and #21, therefore it’s locked
• box #22 is unlocked by workers #1 and #11, and locked by workers #2 and #22, therefore it’s locked
• box #23 is unlocked by worker #1 and locked by worker #23, therefore it’s locked
• box #24 is unlocked by workers #1, #3, #6 and #12, and locked by workers #2, #4, #8 and #24, therefore it’s locked
• box #25 is unlocked by workers #1 and #25, and locked by worker #5, therefore it’s unlocked
• box #26 is unlocked by workers #1 and #13, and locked by workers #2 and #26, therefore it’s locked
• box #27 is unlocked by workers #1 and #9, and locked by workers #3 and #27, therefore it’s locked
• box #28 is unlocked by workers #1, #4 and #14, and locked by workers #2, #7 and #28, therefore it’s locked
• box #29 is unlocked by worker #1 and locked by worker #29, therefore it’s locked
• box #30 is unlocked by workers #1, #3, #6 and #15, and locked by workers #2, #5, #10 and #30, therefore it’s locked
• box #31 is unlocked by worker #1 and locked by worker #31, therefore it’s locked
• box #32 is unlocked by workers #1, #4 and #16, and locked by workers #2, #8 and #32, therefore it’s locked
• box #33 is unlocked by workers #1 and #11, and locked by workers #3 and #33, therefore it’s locked
• box #34 is unlocked by workers #1 and #17, and locked by workers #2 and #34, therefore it’s locked
• box #35 is unlocked by workers #1 and #7, and locked by workers #5 and #35, therefore it’s locked
• box #36 is unlocked by workers #1, #3, #6, #12 and #36, and locked by workers #2, #4, #9 and #18, therefore it’s unlocked
• box #37 is unlocked by worker #1 and locked by worker #37, therefore it’s locked
• box #38 is unlocked by workers #1 and #19, and locked by workers #2 and #38, therefore it’s locked
• box #39 is unlocked by workers #1 and #13, and locked by workers #3 and #39, therefore it’s locked
• box #40 is unlocked by workers #1, #4, #8 and #20, and locked by workers #2, #5, #10 and #40, therefore it’s locked
• box #41 is unlocked by worker #1 and locked by worker #41, therefore it’s locked
• box #42 is unlocked by workers #1, #3, #7 and #21, and locked by workers #2, #6, #14 and #42, therefore it’s locked
• box #43 is unlocked by worker #1 and locked by worker #43, therefore it’s locked
• box #44 is unlocked by workers #1, #4 and #22, and locked by workers #2, #11 and #44, therefore it’s locked
• box #45 is unlocked by workers #1, #5 and #15, and locked by workers #3, #9 and #45, therefore it’s locked
• box #46 is unlocked by workers #1 and #23, and locked by workers #2 and #46, therefore it’s locked
• box #47 is unlocked by worker #1 and locked by worker #47, therefore it’s locked
• box #48 is unlocked by workers #1, #3, #6, #12 and #24, and locked by workers #2, #4, #8, #16 and #48, therefore it’s locked
• box #49 is unlocked by workers #1 and #49, and locked by worker #7, therefore it’s unlocked
• box #50 is unlocked by workers #1, #5 and #25, and locked by workers #2, #10 and #50, therefore it’s locked
• box #51 is unlocked by workers #1 and #17, and locked by workers #3 and #51, therefore it’s locked
• box #52 is unlocked by workers #1, #4 and #26, and locked by workers #2, #13 and #52, therefore it’s locked
• box #53 is unlocked by worker #1 and locked by worker #53, therefore it’s locked
• box #54 is unlocked by workers #1, #3, #9 and #27, and locked by workers #2, #6, #18 and #54, therefore it’s locked
• box #55 is unlocked by workers #1 and #11, and locked by workers #5 and #55, therefore it’s locked
• box #56 is unlocked by workers #1, #4, #8 and #28, and locked by workers #2, #7, #14 and #56, therefore it’s locked
• box #57 is unlocked by workers #1 and #19, and locked by workers #3 and #57, therefore it’s locked
• box #58 is unlocked by workers #1 and #29, and locked by workers #2 and #58, therefore it’s locked
• box #59 is unlocked by worker #1 and locked by worker #59, therefore it’s locked
• box #60 is unlocked by workers #1, #3, #5, #10, #15 and #30, and locked by workers #2, #4, #6, #12, #20 and #60, therefore it’s locked
• box #61 is unlocked by worker #1 and locked by worker #61, therefore it’s locked
• box #62 is unlocked by workers #1 and #31, and locked by workers #2 and #62, therefore it’s locked
• box #63 is unlocked by workers #1, #7 and #21, and locked by workers #3, #9 and #63, therefore it’s locked
• box #64 is unlocked by workers #1, #4, #16 and #64, and locked by workers #2, #8 and #32, therefore it’s unlocked
• box #65 is unlocked by workers #1 and #13, and locked by workers #5 and #65, therefore it’s locked
• box #66 is unlocked by workers #1, #3, #11 and #33, and locked by workers #2, #6, #22 and #66, therefore it’s locked
• box #67 is unlocked by worker #1 and locked by worker #67, therefore it’s locked
• box #68 is unlocked by workers #1, #4 and #34, and locked by workers #2, #17 and #68, therefore it’s locked
• box #69 is unlocked by workers #1 and #23, and locked by workers #3 and #69, therefore it’s locked
• box #70 is unlocked by workers #1, #5, #10 and #35, and locked by workers #2, #7, #14 and #70, therefore it’s locked
• box #71 is unlocked by worker #1 and locked by worker #71, therefore it’s locked
• box #72 is unlocked by workers #1, #3, #6, #9, #18 and #36, and locked by workers #2, #4, #8, #12, #24 and #72, therefore it’s locked
• box #73 is unlocked by worker #1 and locked by worker #73, therefore it’s locked
• box #74 is unlocked by workers #1 and #37, and locked by workers #2 and #74, therefore it’s locked
• box #75 is unlocked by workers #1, #5 and #25, and locked by workers #3, #15 and #75, therefore it’s locked
• box #76 is unlocked by workers #1, #4 and #38, and locked by workers #2, #19 and #76, therefore it’s locked
• box #77 is unlocked by workers #1 and #11, and locked by workers #7 and #77, therefore it’s locked
• box #78 is unlocked by workers #1, #3, #13 and #39, and locked by workers #2, #6, #26 and #78, therefore it’s locked
• box #79 is unlocked by worker #1 and locked by worker #79, therefore it’s locked
• box #80 is unlocked by workers #1, #4, #8, #16 and #40, and locked by workers #2, #5, #10, #20 and #80, therefore it’s locked
• box #81 is unlocked by workers #1, #9 and #81, and locked by workers #3 and #27, therefore it’s unlocked
• box #82 is unlocked by workers #1 and #41, and locked by workers #2 and #82, therefore it’s locked
• box #83 is unlocked by worker #1 and locked by worker #83, therefore it’s locked
• box #84 is unlocked by workers #1, #3, #6, #12, #21 and #42, and locked by workers #2, #4, #7, #14, #28 and #84, therefore it’s locked
• box #85 is unlocked by workers #1 and #17, and locked by workers #5 and #85, therefore it’s locked
• box #86 is unlocked by workers #1 and #43, and locked by workers #2 and #86, therefore it’s locked
• box #87 is unlocked by workers #1 and #29, and locked by workers #3 and #87, therefore it’s locked
• box #88 is unlocked by workers #1, #4, #11 and #44, and locked by workers #2, #8, #22 and #88, therefore it’s locked
• box #89 is unlocked by worker #1 and locked by worker #89, therefore it’s locked
• box #90 is unlocked by workers #1, #3, #6, #10, #18 and #45, and locked by workers #2, #5, #9, #15, #30 and #90, therefore it’s locked
• box #91 is unlocked by workers #1 and #13, and locked by workers #7 and #91, therefore it’s locked
• box #92 is unlocked by workers #1, #4 and #46, and locked by workers #2, #23 and #92, therefore it’s locked
• box #93 is unlocked by workers #1 and #31, and locked by workers #3 and #93, therefore it’s locked
• box #94 is unlocked by workers #1 and #47, and locked by workers #2 and #94, therefore it’s locked
• box #95 is unlocked by workers #1 and #19, and locked by workers #5 and #95, therefore it’s locked
• box #96 is unlocked by workers #1, #3, #6, #12, #24 and #48, and locked by workers #2, #4, #8, #16, #32 and #96, therefore it’s locked
• box #97 is unlocked by worker #1 and locked by worker #97, therefore it’s locked
• box #98 is unlocked by workers #1, #7 and #49, and locked by workers #2, #14 and #98, therefore it’s locked
• box #99 is unlocked by workers #1, #9 and #33, and locked by workers #3, #11 and #99, therefore it’s locked
• box #100 is unlocked by workers #1, #4, #10, #25 and #100, and locked by workers #2, #5, #20 and #50, therefore it’s unlocked

Piles of Prime Pairs

by in Mathematics, Prime numbers and tagged , , , , | Leave a comment

A087641 Start of the first sequence of exactly n consecutive pairs of twin primes

29, 101, 5, 9419, 909287, 325267931, 678771479, 1107819732821, 170669145704411, 3324648277099157, 789795449254776509

Example: a(6)=325267931 is the starting point of the first occurrence of 6 consecutive pairs of twin primes: (325267931 325267933) (325267937 325267939) (325267949 325267951) (325267961 325267963) (325267979 325267981) (325267991 325267993).

• A087641 at the Encyclopedia of Integer Sequences

Abounding in Abundants

by in Arithmetic, Integer Sequences, Mathematics, Prime numbers and tagged , , , , , , , , , , , , , , , , | Leave a comment

This is the famous Ulam spiral, invented by the Jewish mathematician Stanisław Ulam (pronounced OO-lam) to represent prime numbers on a square grid:

The Ulam spiral of prime numbers

The red square represents 1, with 2 as the white block immediately to its right and 3 immediately above 2. Then 5 is the white block one space to the left of 3 and 7 the white block one space below 5. Then 11 is the white block right beside 2 and 13 the white block one space above 11. And so on. The primes aren’t regularly spaced on the spiral but patterns are nevertheless appearing. Here’s the Ulam spiral at higher resolutions:

The Ulam spiral x2

The Ulam spiral x4

The primes are neither regular nor random in their distribution on the spiral. They stand tantalizingly betwixt and between. So the numbers represented on this Ulam-like spiral, which looks like an aerial view of a city designed by architects who occasionally get drunk:

Ulam-like spiral of abundant numbers

The distribution of abundant numbers is much more regular than the primes, but is far from wholly predictable. And what are abundant numbers? They’re numbers n such that sum(divisors(n)-n) > n. In other words, when you add the divisors of n less than n, the sum is greater than n. The first abundant number is 12:

12 is divisible by 1, 2, 3, 4, 6 → 1 + 2 + 3 + 4 + 6 = 16 > 12

The abundant numbers go like this:

12, 18, 20, 24, 30, 36, 40, 42, 48, 54, 56, 60, 66, 70, 72, 78, 80, 84, 88, 90, 96, 100, 102, 104, 108, 112, 114, 120, 126, 132, 138, 140, 144, 150, 156, 160, 162, 168, 174, 176, 180, 186, 192, 196, 198, 200, 204, 208, 210, 216, 220, 222, 224, 228, 234, 240, 246, 252, 258, 260, 264, 270… — A005101 at the Online Encyclopedia of Integer Sequences

Are all abundant numbers even? No, but the first odd abundant number takes a long time to arrive: it’s 45045. The abundance of 45045 was first discovered by the French mathematician Charles de Bovelles or Carolus Bovillus (c. 1475-1566), according to David Wells in his wonderful Penguin Dictionary of Curious and Interesting Numbers (1986):

45045 = 3^2 * 5 * 7 * 11 * 13 → 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 21 + 33 + 35 + 39 + 45 + 55 + 63 + 65 + 77 + 91 + 99 + 105 + 117 + 143 + 165 + 195 + 231 + 273 + 315 + 385 + 429 + 455 + 495 + 585 + 693 + 715 + 819 + 1001 + 1155 + 1287 + 1365 + 2145 + 3003 + 3465 + 4095 + 5005 + 6435 + 9009 + 15015 = 59787 > 45045

Here’s the spiral of abundant numbers at higher resolutions:

Abundant numbers x2

Abundant numbers x4

Negating the spiral of the abundant numbers — almost — is the spiral of the deficient numbers, where sum(divisors(n)-n) < n. Like most odd numbers, 15 is deficient:

15 = 3 * 5 → 1 + 3 + 5 = 9 < 15

Here’s the spiral of deficient numbers at various resolutions:

Deficient numbers on Ulam-like spiral

Deficient numbers x2

Deficient numbers x4

The spiral of deficient numbers doesn’t quite negate (reverse the colors of) the spiral of abundant numbers because of the very rare perfect numbers, where sum(divisors(n)-n) = n. That is, their factor-sums are exactly equal to themselves:

• 6 = 2 * 3 → 1 + 2 + 3 = 6
• 28 = 2^2 * 7 → 1 + 2 + 4 + 7 + 14 = 28
• 496 = 2^4 * 31 → 1 + 2 + 4 + 8 + 16 + 31 + 62 + 124 + 248 = 496

Now let’s try numbers n such than sum(divisors(n)) mod 2 = 1 (“n mod 2″ gives the remainder when n is divided by 2, i.e. n mod 2 is either 0 or 1). For example:

• 4 = 2^2 → 1 + 2 + 4 = 7 → 7 mod 2 = 1
• 18 = 2 * 3^2 → 1 + 2 + 3 + 6 + 9 + 18 = 39 → 39 mod 2 = 1
• 72 = 2^3 * 3^2 → 1 + 2 + 3 + 4 + 6 + 8 + 9 + 12 + 18 + 24 + 36 + 72 = 195 → 195 mod 2 = 1

Here are spirals for these numbers:

Ulam-like spiral for n such than sum(divisors(n)) mod 2 = 1

sum(divisors(n)) mod 2 = 1 x2

sum(divisors(n)) mod 2 = 1 x4

sum(divisors(n)) mod 2 = 1 x8

sum(divisors(n)) mod 2 = 1 x16

Summer Samer

by in Arithmetic, Mathematics, Prime numbers and tagged , , , , , , , | Leave a comment

10 can be represented in exactly 10 ways as a sum of distinct integers:


10 = 1 + 2 + 3 + 4
10 = 2 + 3 + 5
10 = 1 + 4 + 5
10 = 1 + 3 + 6
10 = 4 + 6 (c=5)
10 = 1 + 2 + 7
10 = 3 + 7
10 = 2 + 8
10 = 1 + 9
10 = 10 (c=10)

But there’s something unsatisfying about including 10 as a sum of itself. It’s much more satisfying that 76 can be represented in exactly 76 ways as a sum of distinct primes:


76 = 2 + 3 + 7 + 11 + 13 + 17 + 23
76 = 5 + 7 + 11 + 13 + 17 + 23
76 = 2 + 3 + 5 + 11 + 13 + 19 + 23
76 = 3 + 7 + 11 + 13 + 19 + 23
76 = 2 + 3 + 5 + 7 + 17 + 19 + 23 (c=5)
76 = 2 + 3 + 5 + 7 + 13 + 17 + 29
76 = 2 + 3 + 5 + 7 + 11 + 19 + 29
76 = 3 + 5 + 7 + 13 + 19 + 29
76 = 11 + 17 + 19 + 29
76 = 11 + 13 + 23 + 29 (c=10)
76 = 2 + 5 + 17 + 23 + 29
76 = 7 + 17 + 23 + 29
76 = 2 + 3 + 19 + 23 + 29
76 = 5 + 19 + 23 + 29
76 = 2 + 3 + 5 + 7 + 11 + 17 + 31 (c=15)
76 = 3 + 5 + 7 + 13 + 17 + 31
76 = 3 + 5 + 7 + 11 + 19 + 31
76 = 2 + 11 + 13 + 19 + 31
76 = 2 + 7 + 17 + 19 + 31
76 = 2 + 7 + 13 + 23 + 31 (c=20)
76 = 2 + 3 + 17 + 23 + 31
76 = 5 + 17 + 23 + 31
76 = 3 + 19 + 23 + 31
76 = 2 + 3 + 11 + 29 + 31
76 = 5 + 11 + 29 + 31 (c=25)
76 = 3 + 13 + 29 + 31
76 = 3 + 5 + 7 + 11 + 13 + 37
76 = 2 + 7 + 13 + 17 + 37
76 = 2 + 7 + 11 + 19 + 37
76 = 2 + 5 + 13 + 19 + 37 (c=30)
76 = 7 + 13 + 19 + 37
76 = 3 + 17 + 19 + 37
76 = 2 + 3 + 11 + 23 + 37
76 = 5 + 11 + 23 + 37
76 = 3 + 13 + 23 + 37 (c=35)
76 = 2 + 3 + 5 + 29 + 37
76 = 3 + 7 + 29 + 37
76 = 3 + 5 + 31 + 37
76 = 2 + 5 + 11 + 17 + 41
76 = 7 + 11 + 17 + 41 (c=40)
76 = 2 + 3 + 13 + 17 + 41
76 = 5 + 13 + 17 + 41
76 = 2 + 3 + 11 + 19 + 41
76 = 5 + 11 + 19 + 41
76 = 3 + 13 + 19 + 41 (c=45)
76 = 2 + 3 + 7 + 23 + 41
76 = 5 + 7 + 23 + 41
76 = 2 + 7 + 11 + 13 + 43
76 = 2 + 3 + 11 + 17 + 43
76 = 5 + 11 + 17 + 43 (c=50)
76 = 3 + 13 + 17 + 43
76 = 2 + 5 + 7 + 19 + 43
76 = 3 + 11 + 19 + 43
76 = 2 + 3 + 5 + 23 + 43
76 = 3 + 7 + 23 + 43 (c=55)
76 = 2 + 31 + 43
76 = 2 + 3 + 11 + 13 + 47
76 = 5 + 11 + 13 + 47
76 = 2 + 3 + 7 + 17 + 47
76 = 5 + 7 + 17 + 47 (c=60)
76 = 2 + 3 + 5 + 19 + 47
76 = 3 + 7 + 19 + 47
76 = 29 + 47
76 = 2 + 3 + 7 + 11 + 53
76 = 5 + 7 + 11 + 53 (c=65)
76 = 2 + 3 + 5 + 13 + 53
76 = 3 + 7 + 13 + 53
76 = 23 + 53
76 = 2 + 3 + 5 + 7 + 59
76 = 17 + 59 (c=70)
76 = 3 + 5 + 7 + 61
76 = 2 + 13 + 61
76 = 2 + 7 + 67
76 = 2 + 3 + 71
76 = 5 + 71 (c=75)
76 = 3 + 73

Power Flip

by in Arithmetic, Base Behavior, Integer Sequences, Mathematics, Powers, Prime numbers and tagged , , , , , , , , , , , , , , | Leave a comment

12 is an interesting number in a lot of ways. Here’s one way I haven’t seen mentioned before:

12 = 3^1 * 2^2


The digits of 12 represent the powers of the primes in its factorization, if primes are represented from right-to-left, like this: …7, 5, 3, 2. But I couldn’t find any more numbers like that in base 10, so I tried a power flip, from right-left to left-right. If the digits from left-to-right represent the primes in the order 2, 3, 5, 7…, then this number is has prime-power digits too:

81312000 = 2^8 * 3^1 * 5^3 * 7^1 * 11^2 * 13^0 * 17^0 * 19^0


Or, more simply, given that n^0 = 1:

81312000 = 2^8 * 3^1 * 5^3 * 7^1 * 11^2


I haven’t found any more left-to-right prime-power digital numbers in base 10, but there are more in other bases. Base 5 yields at least three (I’ve ignored numbers with just two digits in a particular base):

110 in b2 = 2^1 * 3^1 (n=6)
130 in b6 = 2^1 * 3^3 (n=54)
1010 in b2 = 2^1 * 3^0 * 5^1 (n=10)
101 in b3 = 2^1 * 3^0 * 5^1 (n=10)
202 in b7 = 2^2 * 3^0 * 5^2 (n=100)
3020 in b4 = 2^3 * 3^0 * 5^2 (n=200)
330 in b8 = 2^3 * 3^3 (n=216)
13310 in b14 = 2^1 * 3^3 * 5^3 * 7^1 (n=47250)
3032000 in b5 = 2^3 * 3^0 * 5^3 * 7^2 (n=49000)
21302000 in b5 = 2^2 * 3^1 * 5^3 * 7^0 * 11^2 (n=181500)
7810000 in b9 = 2^7 * 3^8 * 5^1 (n=4199040)
81312000 in b10 = 2^8 * 3^1 * 5^3 * 7^1 * 11^2


Post-Performative Post-Scriptum

When I searched for 81312000 at the Online Encyclopedia of Integer Sequences, I discovered that these are Meertens numbers, defined at A246532 as the “base n Godel encoding of x [namely,] 2^d(1) * 3^d(2) * … * prime(k)^d(k), where d(1)d(2)…d(k) is the base n representation of x.”