Agogic Arithmetic

Thursday, 29th Dec, 2022 by Krilling for Company in Base Behavior, Integer sequences, Mathematics, Triangular Numbers and tagged harmonic numbers, harmonic series, leading digits, number sequences, OEIS, Online Encyclopedia of Integer Sequences | Leave a comment

This is one of my favorite integer sequences:
• 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406, 435, 465, 496, 528, 561, 595, 630, 666, 703, 741, 780, 820, 861, 903, 946, 990, 1035, 1081, 1128, 1176, 1225, 1275, 1326, 1378, 1431, ... — A000217 at OEIS

And it’s easy to work out the rule that generates the sequence. It’s the sequence of triangular numbers, of course, which you get by summing the integers:
1 1 + 2 = 3 3 + 3 = 6 6 + 4 = 10 10 + 5 = 15 15 + 6 = 21 21 + 7 = 28 28 + 8 = 36 36 + 9 = 45 [...]

I like this sequence too, but it isn’t a sequence of integers and it’s much harder to work out the rule that generates it:
• 1, 3/2, 11/6, 25/12, 137/60, 49/20, 363/140, 761/280, 7129/2520, 7381/2520, 83711/27720, 86021/27720, 1145993/360360, 1171733/360360...

But you could say that it’s the inverse of the triangular numbers, because you generate it like this:
1 1 + 1/2 = 3/2 3/2 + 1/3 = 11/6 11/6 + 1/4 = 25/12 25/12 + 1/5 = 137/60 137/60 + 1/6 = 49/20 49/20 + 1/7 = 363/140 363/140 + 1/8 = 761/280 761/280 + 1/9 = 7129/2520 [...]
It’s the harmonic series, which is defined at Wikipedia as “the infinite series formed by summing all positive unit fractions”. I can’t understand its subtleties or make any important discoveries about it, but I thought I could ask (and begin to answer) a question that perhaps no-one else in history had ever asked: When are the leading digits of the k-th harmonic number, hs(k), equal to the digits of k in base 10?
hs(1) = 1 hs(43) = 4.349... hs(714) = 7.1487... hs(715) = 7.1501... hs(9763) = 9.76362... hs(122968) = 12.296899... hs(122969) = 12.296907... hs(1478366) = 14.7836639... hs(17239955) = 17.23995590... hs(196746419) = 19.6746419... hs(2209316467) = 22.0931646788...

Do those numbers have any true mathematical significance? I doubt it. But they were fun to find, even though I wasn’t the first person in history to ask about them:
• 1, 43, 714, 715, 9763, 122968, 122969, 1478366, 17239955, 196746419, 2209316467, 24499118645, 268950072605 — A337904 at OEIS, Numbers k such that the decimal expansion of the k-th harmonic number starts with the digits of k, in the same order.

Magiciprocal

Thursday, 19th May, 2022 by Krilling for Company in Base Behavior, Integer sequences, Magic squares, Mathematics, Prime numbers, Reciprocals and tagged 19, magic squares, nineteen, OEIS, Online Encyclopedia of Integer Sequences, primes, recreational math, recreational mathematics, recreational maths | Leave a comment

A021023 Decimal expansion of 1/19.

0, 5, 2, 6, 3, 1, 5, 7, 8, 9, 4, 7, 3, 6, 8, 4, 2, 1, 0, 5, 2, 6, 3, 1, 5, 7, 8, 9, 4, 7, 3, 6, 8, 4, 2, 1, 0, 5, 2, 6, 3, 1, 5, 7, 8, 9, 4, 7, 3, 6, 8, 4, 2, 1, 0, 5, 2, 6, 3, 1, 5, 7, 8, 9, 4, 7, 3, 6, 8, 4, 2, 1, 0, 5, 2, 6, 3, 1, 5, 7, 8, 9, 4, 7, 3, 6, 8, 4, 2, 1, 0, 5, 2, 6, 3, 1, 5, 7, 8 [...] The magic square that uses the decimals of 1/19 is fully magic. — A021023 at the Online Encyclopedia of Integer Sequences

Mötley Vüe

Sunday, 23rd Jan, 2022 by Krilling for Company in Base Behavior, Digit-sums, Fibonacci sequence, Integer sequences, φ, Mathematics and tagged David C. Terr, David Terr, digit-sums, digsum, Fibonacci numbers, Fibonacci Quarterly, Fibonacci sequence, integer sequences, math, mathematics, maths, OEIS, Online Encyclopedia of Integer Sequences, recreational math, recreational mathematics, recreational maths | Leave a comment

Here’s the Fibonacci sequence, where each term (after the first two) is created by adding the two previous numbers:

1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765...

In “Fib and Let Tri”, I described how my eye was caught by 55, which is a palindrome, reading the same backwards and forwards. “Were there any other Fibonacci palindromes?” I wondered. So I looked to see. Now my eye has been caught by 55 again, but for another reason. It should be easy to spot another interesting aspect to 55 when the Fibonacci numbers are set out like this:

fib(1) = 1 fib(2) = 1 fib(3) = 2 fib(4) = 3 fib(5) = 5 fib(6) = 8 fib(7) = 13 fib(8) = 21 fib(9) = 34 fib(10) = 55 fib(11) = 89 fib(12) = 144 fib(13) = 233 fib(14) = 377 fib(15) = 610 fib(16) = 987 fib(17) = 1597 fib(18) = 2584 fib(19) = 4181 fib(20) = 6765 [...]

55 is fib(10), the 10th Fibonacci number, and 5+5 = 10. That is, digsum(fib(10)) = 10. What other Fibonacci numbers work like that? I soon found some and confirmed my answer at the Online Encyclopedia of Integer Sequences:

1, 5, 10, 31, 35, 62, 72, 175, 180, 216, 251, 252, 360, 494, 504, 540, 946, 1188, 2222 — A020995 at OEIS

And that seems to be the lot, according to the OEIS. In base 10, at least, but why stop at base 10? When I looked at base 11, the numbers of digsum(fib(k)) = k didn’t stop coming, because I couldn’t take the Fibonacci numbers very high on my computer. But the OEIS gives a much longer list, starting like this:

1, 5, 13, 41, 53, 55, 60, 61, 90, 97, 169, 185, 193, 215, 265, 269, 353, 355, 385, 397, 437, 481, 493, 617, 629, 630, 653, 713, 750, 769, 780, 889, 905, 960, 1013, 1025, 1045, 1205, 1320, 1405, 1435, 1501, 1620, 1650, 1657, 1705, 1735, 1769, 1793, 1913, 1981, 2125, 2153, 2280, 2297, 2389, 2413, 2460, 2465, 2509, 2533, 2549, 2609, 2610, 2633, 2730, 2749, 2845, 2893, 2915, 3041, 3055, 3155, 3209, 3360, 3475, 3485, 3521, 3641, 3721, 3749, 3757, 3761, 3840, 3865, 3929, 3941, 4075, 4273, 4301, 4650, 4937, 5195, 5209, 5435, 5489, 5490, 5700, 5917, 6169, 6253, 6335, 6361, 6373, 6401, 6581, 6593, 6701, 6750, 6941, 7021, 7349, 7577, 7595, 7693, 7740, 7805, 7873, 8009, 8017, 8215, 8341, 8495, 8737, 8861, 8970, 8995, 9120, 9133, 9181, 9269, 9277, 9535, 9541, 9737, 9935, 9953, 10297, 10609, 10789, 10855, 11317, 11809, 12029, 12175... — A020995 at OEIS

The list ends with 1636597 = A18666[b11] and the OEIS says that 1636597 almost certainly completes the list. According to David C. Terr’s paper “On the Sums of Fibonacci Numbers” (pdf), published in the Fibonacci Quarterly in 1996, the estimated digit-sum for the k-th Fibonacci number in base b is given by the formula (b-1)/2 * k * log(b,φ), where log(b,φ) is the logarithm in base b of the golden ratio, 1·61803398874… Terr then notes that the simplified formula (b-1)/2 * log(b,φ) gives the estimated average ratio digsum(fib(k)) / k in base b. Here are the estimates for bases 2 to 20:

b02 = 0.3471209568153086... b03 = 0.4380178794859424... b04 = 0.5206814352229629... b05 = 0.5979874356654401... b06 = 0.6714235829697111... b07 = 0.7418818776805580... b08 = 0.8099488992357201... b09 = 0.8760357589718848... b10 = 0.9404443811249043... b11 = 1.0034045909311624... b12 = 1.0650963641043091... b13 = 1.1256639207937723... b14 = 1.1852250528196852... b15 = 1.2438775226715552... b16 = 1.3017035880574074... b17 = 1.3587732842474014... b18 = 1.4151468584732730... b19 = 1.4708766105122322... b20 = 1.5260083080264088...

In base 2, you can expect digsum(fib(k)) to be much smaller than k; in base 20, you can expect digsum(fib(k)) to be much larger. But as you can see, the estimate for base 11, 1.0034045909311624…, is very nearly 1. That’s why base 11 produces so many results for digsum(fib(k)) = k, because only a slight deviation from the estimate might create a perfect ratio of 1 for digsum(fib(k)) / k, i.e. digsum(fib(k)) = k. But in the end the results run out in base 11 too, because as k gets higher and fib(k) gets bigger, the estimate becomes more and more accurate and digsum(fib(k)) > k. With lower k, digsum(fib(k)) can easily fall below k or match k. That happens in other bases, but because their estimates are further from 1, results for digsum(fib(k)) = k run out much more quickly.

To see this base behavior represented visually, I’ve created Ulam-like spirals for k using three colors: blue for digsum(fib(k)) < k, yellow for digsum(fib(k)) > k, and red for digsum(fib(k)) = k (with the green square at the center representing fib(1) = 1). As you can see below, the spiral for base 11 immediately stands out. It’s motley, not dominated by blue or yellow like the other spirals:

Spiral for digsum(fib(k)) in base 9
(blue for digsum(fib(k)) < k, yellow for digsum(fib(k)) > k, red for digsum(fib(k)) = k, green for fib(1))

Spiral for digsum(fib(k)) in base 10

Spiral for digsum(fib(k)) in base 11 — a motley view of blue, yellow and red

Spiral for digsum(fib(k)) in base 12

Spiral for digsum(fib(k)) in base 13

Finally, here are spirals at higher and higher resolution for digsum(fib(k)) = k in base 11:

digsum(fib(k)) = k in base 11 (low resolution)
(green square is fib(1))

digsum(fib(k)) = k in base 11 (x2 resolution)

digsum(fib(k)) = k in base 11 (x4)

digsum(fib(k)) = k in base 11 (x8)

digsum(fib(k)) = k in base 11 (x16)

digsum(fib(k)) = k in base 11 (x32)

digsum(fib(k)) = k in base 11 (x64)

digsum(fib(k)) = k in base 11 (x128)

digsum(fib(k)) = k in base 11 (animated)

Triangular Squares

Wednesday, 5th Jan, 2022 by Krilling for Company in √2, Integer sequences, Irrational numbers, Mathematics, Square root of 2, Square Roots, Squares, Triangular Numbers and tagged A001110, David Wells, factors, integer sequences, math, mathematics, maths, OEIS, Penguin Dictionary of Curious and Interesting Mathematics, square numbers, square root of 2, square triangular numbers, Squares, Triangular Numbers | Leave a comment

The numbers that are both square and triangular are beautifully related to the best approximations to √2:

Number	Square Root	Factors of root
1	1	1
36	6	2 * 3
1225	35	5 * 7
41616	204	12 * 17

and so on.

In each case the factors of the root are the numerator and denominator of the next approximation to √2. — David Wells, The Penguin Dictionary of Curious and Interesting Mathematics (1986), entry for “36”.

Elsewhere other-accessible

• A001110 — Square triangular numbers: numbers that are both triangular and square

Prime Times

Saturday, 11th Dec, 2021 by Krilling for Company in Integer sequences, Mathematics, Prime numbers and tagged A005234, math, mathematics, maths, OEIS, Online Encylopedia of Integer Sequences, prime numbers, primes, primorials, recreational math, recreational mathematics, recreational maths | Leave a comment

The factorial of an integer is equal to that that integer multiplied by all the integers smaller than it. For example, this is factorial(7) or 7!:
7! = 7 * 6 * 5 * 4 * 3 * 2 * 1 = 5040
The primorial of a prime is equal to that that prime multiplied by all the primes smaller than it. For example, this is primorial(7):
primorial(7) = 7 * 5 * 3 * 2 = 210 = 4# (the product of the first four primes)
Here’s an interesting set of primorials incremented-by-one:
primorial(2) + 1 = 2 + 1 = 3 (prime) primorial(3) + 1 = 2*3 + 1 = 7 (prime) primorial(5) + 1 = 2*3*5 + 1 = 31 (prime) primorial(7) + 1 = 2*3*5*7 + 1 = 211 (prime) primorial(11) + 1 = 2*3*5*7*11 + 1 = 2311 (prime) primorial(31) + 1 = 2*3*5*7*11*13*17*19*23*29*31 + 1 = 200560490131 (prime) primorial(379) + 1 = 1,719,620,105,458,406,433,483,340,568,317,543,019,584,575,635,895,742,560,438,771,105,058,321,655,238,562,613,083,979,651,479,555,788,009,994,557,822,024,565,226,932,906,295,208,262,756,822,275,663,694,111 (prime) primorial(1019) + 1 = 20,404,068,993,016,374,194,542,464,172,774,607,695,659,797,117,423,121,913,227,131,032,339,026,169,175,929,902,244,453,757,410,468,728,842,929,862,271,605,567,818,821,685,490,676,661,985,389,839,958,622,802,465,986,881,376,139,404,138,376,153,096,103,140,834,665,563,646,740,160,279,755,212,317,501,356,863,003,638,612,390,661,668,406,235,422,311,783,742,390,510,526,587,257,026,500,302,696,834,793,248,526,734,305,801,634,165,948,702,506,367,176,701,233,298,064,616,663,553,716,975,429,048,751,575,597,150,417,381,063,934,255,689,124,486,029,492,908,966,644,747,931 (prime) primorial(1021) + 1 = 20,832,554,441,869,718,052,627,855,920,402,874,457,268,652,856,889,007,473,404,900,784,018,145,718,728,624,430,191,587,286,316,088,572,148,631,389,379,309,284,743,016,940,885,980,871,887,083,026,597,753,881,317,772,605,885,038,331,625,282,052,311,121,306,792,193,540,483,321,703,645,630,071,776,168,885,357,126,715,023,250,865,563,442,766,366,180,331,200,980,711,247,645,589,424,056,809,053,468,323,906,745,795,726,223,468,483,433,625,259,000,887,411,959,197,323,973,613,488,345,031,913,058,775,358,684,690,576,146,066,276,875,058,596,100,236,112,260,054,944,287,636,531 (prime) primorial(2657) + 1 = 78,244,737,296,323,701,708,091,142,569,062,680,832,012,147,734,404,650,078,590,391,114,054,859,290,061,421,837,516,998,655,549,776,972,299,461,276,876,623,748,922,539,131,984,799,803,433,363,562,299,977,701,808,549,255,204,262,920,151,723,624,296,938,777,341,738,751,806,450,993,015,446,712,522,509,989,316,673,420,506,749,359,414,629,957,842,716,112,900,306,643,009,542,215,969,000,431,330,219,583,111,410,996,807,066,475,261,560,303,182,609,636,056,108,367,412,324,508,444,341,178,028,289,201,803,518,093,842,982,877,662,621,552,756,279,669,241,303,362,152,895,160,479,720,040,128,335,518,247,125,849,521,099,841,272,983,588,935,580,888,630,036,283,712,163,901,558,436,498,481,482,160,712,530,124,868,714,141,094,634,892,999,056,865,426,200,254,647,241,979,548,935,087,621,308,526,547,138,125,987,102,062,688,568,486,250,939,447,065,798,353,626,745,169,380,579,442,233,006,898 ,444,700,264,240,321,482,823,859,842,044,524,114,576,784,795,294,818,755,525,169,192,652,108,755,230,262,128,210,258,672,754,900,845,837,728,345,782,457,465,793,874,408,469,588,052,577,208,643,754,019,053,756,394,151,041,512,099,598,925,557,724,343,099,264,685,155,934,891,439,161,866,250,113,047,185,553,511,797,406,764,115,907,248,713,405,817,594,729,550,600,082,808,324,331,387,143,679,800,355,356,811,873,430,669,962,333,651,282,822,030,473,702,042,073,141,618,450,021,084,993,659,382,646,598,194,115,178,864,433,545,186,250,667,775,794,249,961,932,761,063,071,117,967,553,887,984,011,652,643,245,393,971 (prime) primorial(3229) + 1 = 689,481,240,122,180,255,681,227,812,346,871,771,457,221,628,238,467,511,261,402,638,443,056,696,165,896,544,725,098,860,107,293,247,422,610,010,824,870,599,655,026,129,367,004,672,337,297,193,288,816,463,520,704,235,722,580,204,218,943,598,425,089,855,869,341,564,771,022,924,163,236,141,415,235,947,085,902,422,536,824,665,765,244,189,167,643,048,218,572,769,125,400,511,177,245,717,452,516,267,205,786,258,497,574,258,715,214,994,129,786,103,824,740,384,634,788,909,041,221,747,073,062,941,769,355,745,272,170,421,584,636,198,911,899,164,272,930,590,704,655,882,680,817,754,473,306,122,122,423,384,160,639,995,940,152,584,830,810,911,265,680,382,263,051,658,031,509,463,010,733,595,465,426,943,956,643,445,876,702,680,730,987,739,513,538,299,069,540,636,616,098,525,527,546,435,002,783,615,353,417,794,625,251,129,892,373,849,727,119,530,335,366,131,575,986,221,685,088,118,143,088,371,896,087,248,659,669,154,564,925,048,225,211,644,681,303,874,490,648,860,319,990,785,185,350,796,853,298,548,942,407,689,617,641,587,755,314,125,485,345,107,782,298,938,892,240,282,038,605,672,241,010,302,874,153,509,795,545,077,305,234,459,038,983,235,361,138,814,897,166,376,363,090,128,647,084,552,385,969,054,439,430,382,421,762,883,708,894,899,853,286,109,068,224,980,793,075,241,538,872,287,253,835,877,394,821,667,363,465,425,187,353,453,157,415,169,810,167,271,517,665,273,484,442,461,468,031,313,956,356,871,467,191,959,110,440,864,194,544,244,079,053,955,897,287,010,339,385,419,923,838,571,256,564,818,350,769,518,898,003,780,557,167,344,272,499,224,580,817,920,441,512,610,104,625,622,872,289,967,615,843,092,782,763,554,732,404,239,287,463,466,833,602,966,629,613,502,579,134,371,295,289,680,374,088,987,611,189,907,873,072,122,808,833,765,972,650,050,982,877,578,244,899,073,193,043,546,490,795,625,023,568,563,926,988,371 (prime)

Elsewhere Other-Accessible

• A005234 at the Online Encylopedia of Integer Sequences — “Primorial plus 1 primes: primes p such that 1 + product of primes up to p is prime”.

Count Amounts

Wednesday, 11th Nov, 2020 by Krilling for Company in Base Behavior, Integer sequences, Mathematics and tagged base 10, base 6, binary, digit-line, digits, integer sequences, math, maths, OEIS, Online Encyclopedia of Integer Sequences, recreational math, recreational mathematics, recreational maths | Leave a comment

One of my favourite integer sequences is what I call the digit-line. You create it by taking this very familiar integer sequence:

• 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20…

And turning it into this one:

• 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 0, 1, 1, 1, 2, 1, 3, 1, 4, 1, 5, 1, 6, 1, 7, 1, 8, 1, 9, 2, 0… (A033307 in the Online Encyclopedia of Integer Sequences)

You simply chop all numbers into single digits. What could be simpler? Well, creating the digit-line couldn’t be simpler, but it is in fact a very complex object. There are hidden depths in its patterns, as even a brief look will uncover. For example, you can try counting the digits as they appear one-by-one in the line and seeing whether the digit-counts compare. Do the 1s of the digit-line always outnumber the 0s, as you might expect? Yes, they do (unless you start the digit-line 0, 1, 2, 3…). But do the 2s always outnumber the 0s? No: at position 2, there’s a 2, and at position 11 there’s a 0. So that’s one 2 and one 0. Does it happen again? Yes, it happens again at the 222nd digit of the digit-line, as below:

1, 2_count=1, 3, 4, 5, 6, 7, 8, 9, 1, 0_count=1, 1, 1, 1, 2₂, 1, 3, 1, 4, 1, 5, 1, 6, 1, 7, 1, 8, 1, 9, 2₃, 0₂, 2₄, 1, 2₅, 2₆, 2₇, 3, 2₈, 4, 2₉, 5, 2₁₀, 6, 2₁₁, 7, 2₁₂, 8, 2₁₃, 9, 3, 0₃, 3, 1, 3, 2₁₄, 3, 3, 3, 4, 3, 5, 3, 6, 3, 7, 3, 8, 3, 9, 4, 0₄, 4, 1, 4, 2₁₅, 4, 3, 4, 4, 4, 5, 4, 6, 4, 7, 4, 8, 4, 9, 5, 0₅, 5, 1, 5, 2₁₆, 5, 3, 5,4, 5, 5, 5, 6, 5, 7, 5, 8, 5, 9, 6, 0₆, 6, 1, 6, 2₁₇, 6, 3, 6, 4, 6, 5, 6, 6, 6, 7, 6, 8, 6, 9, 7, 0₇, 7, 1, 7, 2₁₈, 7, 3, 7, 4, 7, 5, 7, 6, 7, 7, 7, 8, 7, 9, 8, 0₈, 8, 1, 8, 2₁₉, 8, 3, 8, 4, 8, 5, 8, 6, 8, 7, 8, 8, 8, 9, 9, 0₉, 9, 1, 9, 2₂₀, 9, 3, 9, 4, 9, 5, 9, 6, 9, 7, 9, 8, 9, 9, 1, 0₁₀, 0₁₁, 1, 0₁₂, 1, 1, 0₁₃, 2₂₁, 1, 0₁₄, 3, 1, 0₁₅, 4, 1, 0₁₆, 5, 1, 0₁₇, 6, 1, 0₁₈, 7, 1, 0₁₉, 8, 1, 0₂₀, 9, 1, 1, 0₂₁…

So count(2) = count(0) = 1 at digit 11 of the digit-line in the 0 of what was originally 10. And count(2) = count(0) = 21 @ digit 222 in the 0 of what was originally 110. Is a pattern starting to emerge? Yes, it is. Here are the first few points at which the count(2) = count(0) in the digit-line of base 10:
1 @ 11 in 10 21 @ 222 in 110 321 @ 3333 in 1110 4321 @ 44444 in 11110 54321 @ 555555 in 111110 654321 @ 6666666 in 1111110 7654321 @ 77777777 in 11111110 87654321 @ 888888888 in 111111110 987654321 @ 9999999999 in 1111111110 10987654321 @ 111111111110 in 11111111110 120987654321 @ 1222222222221 in 111111111110 [...]
The count(2) = count(0) = 321 at position 3333 in the digit-line, and 4321 at position 44444, and 54321 at position 555555, and so on. I don’t understand why these patterns occur, but you can predict the count-and-position of 2s and 0s easily until position 9999999999, after which things become more complicated. Related patterns for 2 and 0 occur in all other bases except binary (which doesn’t have a 2 digit). Here’s base 6:
1 @ 11 in 10 (1 @ 7 in 6) 21 @ 222 in 110 (13 @ 86 in 42) 321 @ 3333 in 1110 (121 @ 777 in 258) 4321 @ 44444 in 11110 (985 @ 6220 in 1554) 54321 @ 555555 in 111110 (7465 @ 46655 in 9330) 1054321 @ 11111110 in 1111110 (54121 @ 335922 in 55986) 12054321 @ 122222221 in 11111110 (380713 @ 2351461 in 335922) 132054321 @ 1333333332 in 111111110 (2620201 @ 16124312 in 2015538) 1432054321 @ 14444444443 in 1111111110 (17736745 @ 108839115 in 12093234) 15432054321 @ 155555555554 in 11111111110 (118513705 @ 725594110 in 72559410) 205432054321 @ 2111111111105 in 111111111110 (783641641 @ 4788921137 in 435356466) 2205432054321 @ 22222222222220 in 1111111111110 (5137206313 @ 31345665636 in 2612138802)
And what about comparing other pairs of digits? In fact, the count of all digits except 0 matches infinitely often. To write the numbers 1..9 takes one of each digit (except 0). To write the numbers 1 to 99 takes twenty of each digit (except 0). Here’s the proof:

1₁, 2₁, 3₁, 4₁, 5₁, 6₁, 7₁, 8₁, 9₁, 1₂, 0₁, 1₃, 1₄, 1₅, 2₂, 1₆, 3₂, 1₇, 4₂, 1₈, 5₂, 1₉, 6₂, 1₁₀, 7₂, 1₁₁, 8₂, 1₁₂, 9₂, 2₃, 0₂, 2₄, 1₁₃, 2₅, 2₆, 2₇, 3₃, 2₈, 4₃, 2₉, 5₃, 2₁₀, 6₃, 2₁₁, 7₃, 2₁₂, 8₃, 2₁₃, 9₃, 3₄, 0₃, 3₅, 1₁₄, 3₆, 2₁₄, 3₇, 3₈, 3₉, 4₄, 3₁₀, 5₄, 3₁₁, 6₄, 3₁₂, 7₄, 3₁₃, 8₄, 3₁₄, 9₄, 4₅, 0₄, 4₆, 1₁₅, 4₇, 2₁₅, 4₈, 3₁₅, 4₉, 4₁₀, 4₁₁, 5₅, 4₁₂, 6₅, 4₁₃, 7₅, 4₁₄, 8₅, 4₁₅, 9₅, 5₆, 0₅, 5₇, 1₁₆, 5₈, 2₁₆, 5₉, 3₁₆, 5₁₀, 4₁₆, 5₁₁, 5₁₂, 5₁₃, 6₆, 5₁₄, 7₆, 5₁₅, 8₆, 5₁₆, 9₆, 6₇, 0₆, 6₈, 1₁₇, 6₉, 2₁₇, 6₁₀, 3₁₇, 6₁₁, 4₁₇, 6₁₂, 5₁₇, 6₁₃, 6₁₄, 6₁₅, 7₇, 6₁₆, 8₇, 6₁₇, 9₇, 7₈, 0₇, 7₉, 1₁₈, 7₁₀, 2₁₈, 7₁₁, 3₁₈, 7₁₂, 4₁₈, 7₁₃, 5₁₈, 7₁₄, 6₁₈, 7₁₅, 7₁₆, 7₁₇, 8₈, 7₁₈, 9₈, 8₉, 0₈, 8₁₀, 1₁₉, 8₁₁, 2₁₉, 8₁₂, 3₁₉, 8₁₃, 4₁₉, 8₁₄, 5₁₉, 8₁₅, 6₁₉, 8₁₆, 7₁₉, 8₁₇, 8₁₈, 8₁₉, 9₉, 9₁₀, 0₉, 9₁₁, 1₂₀, 9₁₂, 2₂₀, 9₁₃, 3₂₀, 9₁₄, 4₂₀, 9₁₅, 5₂₀, 9₁₆, 6₂₀, 9₁₇, 7₂₀, 9₁₈, 8₂₀, 9₁₉, 9₂₀…

And what about writing 1..999, 1..9999, and so on? If you think about it, for every pair of non-zero digits, d1 and d2, all numbers containing one digit can be matched with a number containing the other. 100 → 200, 111 → 222, 314 → 324, 561189571 → 562289572, and so on. So in counting 1..999, 1..9999, 1..99999, you use the same number of non-zero digits. And once again a pattern emerges:
count(0) = 0; count(1) = 1; count(2) = 1; count(3) = 1; count(4) = 1; count(5) = 1; count(6) = 1; count(7) = 1; count(8) = 1; count(9) = 1 (writing 1..9) count(0) = 9; count(1) = 20; count(2) = 20; count(3) = 20; count(4) = 20; count(5) = 20; count(6) = 20; count(7) = 20; count(8) = 20; count(9) = 20 (writing 1..99) 0: 189; 1: 300; 2: 300; 3: 300; 4: 300; 5: 300; 6: 300; 7: 300; 8: 300; 9: 300 (writing 1..999) 0: 2889; 1: 4000; 2: 4000; 3: 4000; 4: 4000; 5: 4000; 6: 4000; 7: 4000; 8: 4000; 9: 4000 (writing 1..9999) 0: 38889; 1: 50000; 2: 50000; 3: 50000; 4: 50000; 5: 50000; 6: 50000; 7: 50000; 8: 50000; 9: 50000 (writing 1..99999) 0: 488889; 1: 600000; 2: 600000; 3: 600000; 4: 600000; 5: 600000; 6: 600000; 7: 600000; 8: 600000; 9: 600000 (writing 1..999999) 0: 5888889; 1: 7000000; 2: 7000000; 3: 7000000; 4: 7000000; 5: 7000000; 6: 7000000; 7: 7000000; 8: 7000000; 9: 7000000 (writing 1..9999999) [...]
And here’s base 6 again:
0: 0; 1: 1; 2: 1; 3: 1; 4: 1; 5: 1 (writing 1..5) 0: 5; 1: 20; 2: 20; 3: 20; 4: 20; 5: 20 (writing 1..55 in base 6) 0: 145; 1: 300; 2: 300; 3: 300; 4: 300; 5: 300 (writing 1..555) 0: 2445; 1: 4000; 2: 4000; 3: 4000; 4: 4000; 5: 4000 (writing 1..5555) 0: 34445; 1: 50000; 2: 50000; 3: 50000; 4: 50000; 5: 50000 (writing 1..55555) 0: 444445; 1: 1000000; 2: 1000000; 3: 1000000; 4: 1000000; 5: 1000000 (writing 1..555555) 0: 5444445; 1: 11000000; 2: 11000000; 3: 11000000; 4: 11000000; 5: 11000000 (writing 1..5555555) 0: 104444445; 1: 120000000; 2: 120000000; 3: 120000000; 4: 120000000; 5: 120000000 (writing 1..55555555) 0: 1144444445; 1: 1300000000; 2: 1300000000; 3: 1300000000; 4: 1300000000; 5: 1300000000 (writing 1..555555555)

B a Pal

Tuesday, 23rd Jul, 2019 by Krilling for Company in Base Behavior, Binary numbers, Integer sequences, Mathematics, Palindromic numbers and tagged base 2, base 3, binary, double palindromes, integer sequences, integers, OEIS, Online Encyclopedia of Integer Sequences, palindromes, palindromic numbers, ternary | Leave a comment

As a keyly committed core component of the counter-cultural community (I wish!), I like to post especially edgy and esoteric material to Overlord In Terms of Core Issues Around Maximal Engagement with Key Notions of the Über-Feral on the 23rd of each month. And today I may be posting the especially edgiest and esoterickest material ever dot dot dot

After all, this entry at the Online Encyclopedia of Integer Sequences is about numbers that are palindromes in two particularly pertinent bases:

A060792 Numbers that are palindromic in bases 2 and 3.

0, 1, 6643, 1422773, 5415589, 90396755477, 381920985378904469, 1922624336133018996235, 2004595370006815987563563, 8022581057533823761829436662099, 392629621582222667733213907054116073, 32456836304775204439912231201966254787, 428027336071597254024922793107218595973 (A060792 at OEIS, with more entries)

And here are the underlying palindromes:
0: 0 ↔ 0 1: 1 ↔ 1 6643: 1100111110011 ↔ 100010001 1422773: 101011011010110110101 ↔ 2200021200022 5415589: 10100101010001010100101 ↔ 101012010210101 90396755477: 1010100001100000100010000011000010101 ↔ 22122022220102222022122 381920985378904469: 10101001100110110110001110011011001110001101101100110010101 ↔ 2112200222001222121212221002220022112 1922624336133018996235: 11010000011100111000101110001110011011001110001110100011100111000001011 ↔ 122120102102011212112010211212110201201021221 2004595370006815987563563: 110101000011111010101010100101111011110111011110111101001010101010111110000101011 ↔ 221010112100202002120002212200021200202001211010122 8022581057533823761829436662099: 1100101010000100101101110000011011011111111011000011100001101111111101101100000111011010010000101010011 ↔ 21000020210011222122220212010000100001021202222122211001202000012 392629621582222667733213907054116073: 10010111001111000100010100010100000011011011000101011011100000111011010100011011011000000101000101000100011110011101001 ↔ 122102120011102000101101000002010021111120010200000101101000201110021201221 32456836304775204439912231201966254787: 11000011010101111010110010100010010011011010101001101000001000100010000010110010101011011001001000101001101011110101011000011 ↔ 1222100201002211120110022121002012121101011212102001212200110211122001020012221 428027336071597254024922793107218595973: 101000010000000110001000011111100101011110011100001110100011100010001110001011100001110011110101001111110000100011000000010000101 ↔ 222001200110022102121001000200200202022111220202002002000100121201220011002100222

Binary Babushkas

Saturday, 16th Mar, 2019 by Krilling for Company in Base Behavior, Binary numbers, Integer sequences, Mathematics and tagged babushka, babushka numbers, base 2, base 3, base three, base two, binary, binary numbers, grandmother, grandmother numbers, integer sequences, math, mathematics, maths, matryoshka, matryoshka dolls, OEIS, Online Encyclopedia of Integer Sequences, recreational math, recreational mathematics, recreational maths, Russian dolls, ternary, toys | Leave a comment

What’s the connection between grandmothers and this set of numbers?

1, 2, 6, 12, 44, 92, 184, 1208, 1256, 4792, 9912, 19832, 39664, 563952, 576464, 4496112, 4499184, 17996528, 17997488, 143972080, 145057520, 145070832, 294967024, 589944560...

To take the first step towards the answer, you need to put the numbers into binary:

1, 10, 110, 1100, 101100, 1011100, 10111000, 10010111000, 10011101000, 1001010111000, 10011010111000, 100110101111000, 1001101011110000, 10001001101011110000, 10001100101111010000, 10001001001101011110000, 10001001010011011110000, 1000100101001101011110000, 1000100101001111010110000, 1000100101001101011011110000, 1000101001010110011011110000, 1000101001011001101011110000, 10001100101001101011011110000, 100011001010011101011011110000...

The second step is compare those binary numbers with these binary numbers, which represent 1 to 30:

1, 10, 11, 100, 101, 110, 111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111, 10000, 10001, 10010, 10011, 10100, 10101, 10110, 10111, 11000, 11001, 11010, 11011, 11100, 11101, 11110...

To see what’s going on, take the first five numbers from each set:

• 1, 10, 110, 1100, 101100... • 1, 10, 11, 100, 101...

What’s going on? If you look, you can see the n-th binary number of set 1 contains the digits of all binary numbers <= n in set 2. For example, 101100 is the 5th binary number in set 1, so it contains the digits of the binary numbers 1 to 5:

101100 ← 1 101100 ← 10 101100 ← 11 101100 ← 100 101100 ← 101

Now try 1256 = 10,011,101,000, the ninth number in set 1. It contains all the binary numbers from 1 to 1001:

10011101000 ← 1 (n=1) 10011101000 ← 10 (n=2) 10011101000 ← 11 (n=3) 10011101000 ← 100 (n=4) 10011101000 ← 101 (n=5) 10011101000 ← 110 (n=6) 10011101000 ← 111 (n=7) 10011101000 ← 1000 (n=8) 10011101000 ← 1001 (n=9)

But where do grandmothers come in? They come in via this famous toy:

Nested doll or Russian doll

It’s called a Russian doll and the way all the smaller dolls pack inside the largest doll reminds me of the way all the smaller numbers 1 to 1010 pack into 1001010111000. But in the Russian language, as you might expect, Russian dolls aren’t called Russian dolls. Instead, they’re called matryoshki (матрёшки, singular матрёшка), meaning “little matrons”. However, there’s a mistaken idea in English that in Russian they’re called babushka dolls, from Russian бабушка, babuška, meaning “grandmother”. And that’s what I thought, until I did a little research.

But the mistake is there, so I’ll call these babushka numbers or grandmother numbers:

1, 2, 6, 12, 44, 92, 184, 1208, 1256, 4792, 9912, 19832, 39664, 563952, 576464, 4496112, 4499184, 17996528, 17997488, 143972080, 145057520, 145070832, 294967024, 589944560...

They’re sequence A261467 at the Online Encyclopedia of Integer Sequences. They go on for ever, but the biggest known so far is 589,944,560 = 100,011,001,010,011,101,011,011,110,000 in binary. And here is that binary babushka with its binary babies:

100011001010011101011011110000 ← 1 (n=1) 100011001010011101011011110000 ← 10 (n=2) 100011001010011101011011110000 ← 11 (n=3) 100011001010011101011011110000 ← 100 (n=4) 100011001010011101011011110000 ← 101 (n=5) 100011001010011101011011110000 ← 110 (n=6) 100011001010011101011011110000 ← 111 (n=7) 100011001010011101011011110000 ← 1000 (n=8) 100011001010011101011011110000 ← 1001 (n=9) 100011001010011101011011110000 ← 1010 (n=10) 100011001010011101011011110000 ← 1011 (n=11) 100011001010011101011011110000 ← 1100 (n=12) 100011001010011101011011110000 ← 1101 (n=13) 100011001010011101011011110000 ← 1110 (n=14) 100011001010011101011011110000 ← 1111 (n=15) 100011001010011101011011110000 ← 10000 (n=16) 100011001010011101011011110000 ← 10001 (n=17) 100011001010011101011011110000 ← 10010 (n=18) 100011001010011101011011110000 ← 10011 (n=19) 100011001010011101011011110000 ← 10100 (n=20) 100011001010011101011011110000 ← 10101 (n=21) 100011001010011101011011110000 ← 10110 (n=22) 100011001010011101011011110000 ← 10111 (n=23) 100011001010011101011011110000 ← 11000 (n=24) 100011001010011101011011110000 ← 11001 (n=25) 100011001010011101011011110000 ← 11010 (n=26) 100011001010011101011011110000 ← 11011 (n=27) 100011001010011101011011110000 ← 11100 (n=28) 100011001010011101011011110000 ← 11101 (n=29) 100011001010011101011011110000 ← 11110 (n=30)

Babushka numbers exist in higher bases, of course. Here are the first thirteen in base 3 or ternary:

1 contains 1 (c=1) (n=1) 12 contains 1, 2 (c=2) (n=5) 102 contains 1, 2, 10 (c=3) (n=11) 1102 contains 1, 2, 10, 11 (c=4) (n=38) 10112 contains 1, 2, 10, 11, 12 (c=5) (n=95) 101120 contains 1, 2, 10, 11, 12, 20 (c=6) (n=285) 1021120 contains 1, 2, 10, 11, 12, 20, 21 (c=7) (n=933) 10211220 contains 1, 2, 10, 11, 12, 20, 21, 22 (c=8) (n=2805) 100211220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100 (c=9) (n=7179) 10021011220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100, 101 (c=10) (n=64284) 1001010211220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100, 101, 102 (c=11) (n=553929) 1001011021220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100, 101, 102, 110 (c=12) (n=554253) 10010111021220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100, 101, 102, 110, 111 (c=13) (n=1663062)

Look at 1,001,010,211,220 (n=553929) and 1,001,011,021,220 (n=554253). They have the same number of digits, but the babushka 1,001,011,021,220 manages to pack in one more baby:

1001010211220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100, 101, 102 (c=11) (n=553929) 1001011021220 contains 1, 2, 10, 11, 12, 20, 21, 22, 100, 101, 102, 110 (c=12) (n=554253)

That happens in binary too:

10010111000 contains 1, 10, 11, 100, 101, 110, 111, 1000, 1001 (c=9) (n=1208) 10011101000 contains 1, 10, 11, 100, 101, 110, 111, 1000, 1001, 1010 (c=10) (n=1256)

What happens in higher bases? Watch this space.

Pi and By

Wednesday, 19th Dec, 2018 by Krilling for Company in √2, Base Behavior, Binary numbers, Integer sequences, Irrational numbers, π, Mathematics, Pi, Square root of 2 and tagged 2, 572, 59609420837337474, 8410815, A049364, √2, digits, integers, irrational numbers, π, math, mathematics, maths, normal numbers, OEIS, Online Encyclopedia of Integer Sequences, recreational math, recreational mathematics, recreational maths, transcendental numbers | Leave a comment

Here’s √2 in base 2:

√2 = 1.01101010000010011110... (base=2)

And in base 3:

√2 = 1.10201122122200121221... (base=3)

And in bases 4, 5, 6, 7, 8, 9 and 10:

√2 = 1.12220021321212133303... (b=4) √2 = 1.20134202041300003420... (b=5) √2 = 1.22524531420552332143... (b=6) √2 = 1.26203454521123261061... (b=7) √2 = 1.32404746317716746220... (b=8) √2 = 1.36485805578615303608... (b=9) √2 = 1.41421356237309504880... (b=10)

And here’s π in the same bases:

π = 11.00100100001111110110... (b=2) π = 10.01021101222201021100... (b=3) π = 03.02100333122220202011... (b=4) π = 03.03232214303343241124... (b=5) π = 03.05033005141512410523... (b=6) π = 03.06636514320361341102... (b=7) π = 03.11037552421026430215... (b=8) π = 03.12418812407442788645... (b=9) π = 03.14159265358979323846... (b=10)

Mathematicians know that in all standard bases, the digits of √2 and π go on for ever, without falling into any regular pattern. These numbers aren’t merely irrational but transcedental. But are they also normal? That is, in each base b, do the digits 0 to [b-1] occur with the same frequency 1/b? (In general, a sequence of length l will occur in a normal number with frequency 1/(b^l).) In base 2, are there as many 1s as 0s in the digits of √2 and π? In base 3, are there as many 2s as 1s and 0s? And so on.

It’s a simple question, but so far it’s proved impossible to answer. Another question starts very simple but quickly gets very difficult. Here are the answers so far at the Online Encyclopedia of Integer Sequences (OEIS):

2, 572, 8410815, 59609420837337474 – A049364

The sequence is defined as the “Smallest number that is digitally balanced in all bases 2, 3, … n”. In base 2, the number 2 is 10, which has one 1 and one 0. In bases 2 and 3, 572 = 1000111100 and 210012, respectively. 1000111100 has five 1s and five 0s; 210012 has two 2s, two 1s and two 0s. Here are the numbers of A049364 in the necessary bases:

10 (n=2) 1000111100, 210012 (n=572) 100000000101011010111111, 120211022110200, 200011122333 (n=8410815) 11010011110001100111001111010010010001101011100110000010, 101201112000102222102011202221201100, 3103301213033102101223212002, 1000001111222333324244344 (n=59609420837337474)

But what number, a(6), satisfies the definition for bases 2, 3, 4, 5 and 6? According to the notes at the OEIS, a(6) > 5^434. That means finding a(6) is way beyond the power of present-day computers. But I assume a quantum computer could crack it. And maybe someone will come up with a short-cut or even an algorithm that supplies a(b) for any base b. Either way, I think we’ll get there, π and by.

Gyp Cip

Saturday, 29th Sep, 2018 by Krilling for Company in Integer sequences, Mathematics, Reciprocals and tagged Egyptian fractions, fractions, integer sequences, OEIS, Online Encyclopedia of Integer Sequences, reciprocals, recreational math, recreational mathematics, recreational maths, sum of fractions, sum of reciprocals, sum of reciprocals to unity, summing fractions, summing reciprocals, unit fractions | Leave a comment

Abundance often overwhelms, but restriction reaps riches. That’s true in mathematics and science, where you can often understand the whole better by looking at only a part of it first — restriction reaps riches. Egyptian fractions are one example in maths. In ancient Egypt, you could have any kind of fraction you liked so long as it was a reciprocal like 1/2, 1/3, 1/4 or 1/5 (well, there were two exceptions: 2/3 and 3/4 were also allowed).

So when mathematicians speak of “Egyptian fractions”, they mean those fractions that can be represented as a sum of reciprocals. Egyptian fractions are restricted and that reaps riches. Here’s one example: how many ways can you add n distinct reciprocals to make 1? When n = 1, there’s one way to do it: 1/1. When n = 2, there’s no way to do it, because 1 – 1/2 = 1/2. Therefore the summed reciprocals aren’t distinct: 1/2 + 1/2 = 1. After that, 1 – 1/3 = 2/3, 1 – 1/4 = 3/4, and so on. By the modern meaning of “Egyptian fraction”, there’s no solution for n = 2.

However, when n = 3, there is a way to do it:
• 1/2 + 1/3 + 1/6 = 1
But that’s the only way. When n = 4, things get better:
• 1/2 + 1/4 + 1/6 + 1/12 = 1 • 1/2 + 1/3 + 1/10 + 1/15 = 1 • 1/2 + 1/3 + 1/9 + 1/18 = 1 • 1/2 + 1/4 + 1/5 + 1/20 = 1 • 1/2 + 1/3 + 1/8 + 1/24 = 1 • 1/2 + 1/3 + 1/7 + 1/42 = 1
What about n = 5, n = 6 and so on? You can find the answer at the Online Encyclopedia of Integer Sequences (OEIS), where sequence A006585 is described as “Egyptian fractions: number of solutions to 1 = 1/x₁ + … + 1/x_n in positive integers x₁ < … < x_n”. The sequence is one of the shortest and strangest at the OEIS:
• 1, 0, 1, 6, 72, 2320, 245765, 151182379
When n = 1, there’s one solution: 1/1. When n = 2, there’s no solution, as I showed above. When n = 3, there’s one solution again. When n = 4, there are six solutions. And the OEIS tells you how many solutions there are for n = 5, 6, 7, 8. But n >= 9 remains unknown at the time of writing.

To understand the problem, consider the three reciprocals, 1/2, 1/3 and 1/5. How do you sum them? They have different denominators, 2, 3 and 5, so you have to create a new denominator, 30 = 2 * 3 * 5. Then you have to adjust the numerators (the numbers above the fraction bar) so that the new fractions have the same value as the old:
• 1/2 = 15/30 = (2*3*5 / 2) / 30 • 1/3 = 10/30 = (2*3*5 / 3) / 30 • 1/5 = 06/30 = (2*3*5 / 5) / 30 • 15/30 + 10/30 + 06/30 = (15+10+6) / 30 = 31/30 = 1 + 1/30
Those three reciprocals don’t sum to 1. Now try 1/2, 1/3 and 1/6:
• 1/2 = 18/36 = (2*3*6 / 2) / 36 • 1/3 = 12/36 = (2*3*6 / 3) / 36 • 1/6 = 06/36 = (2*3*6 / 6) / 36 • 18/36 + 12/36 + 06/36 = (18+12+6) / 36 = 36/36 = 1
So when n = 3, the problem consists of finding three reciprocals, 1/a, 1/b and 1/c, such that for a, b, and c:
• a*b*c = a*b + a*c + b*c
There is only one solution: a = 2, b = 3 and c = 6. When n = 4, the problem consists of finding four reciprocals, 1/a, 1/b, 1/c and 1/d, such that for a, b, c and d:
• a*b*c*d = a*b*c + a*b*d + a*c*d + b*c*d
For example:
• 2*4*6*12 = 576 • 2*4*6 + 2*4*12 + 2*6*12 + 4*6*12 = 48 + 96 + 144 + 288 = 576 • 2*4*6*12 = 2*4*6 + 2*4*12 + 2*6*12 + 4*6*12 = 576
Therefore:
• 1/2 + 1/4 + 1/6 + 1/12 = 1
When n = 5, the problem consists of finding five reciprocals, 1/a, 1/b, 1/c, 1/d and 1/e, such that for a, b, c, d and e:
• a*b*c*d*e = a*b*c*d + a*b*c*e + a*b*d*e + a*c*d*e + b*c*d*e
There are 72 solutions and here they are:
• 1/2 + 1/4 + 1/10 + 1/12 + 1/15 = 1 (#1) • 1/2 + 1/4 + 1/9 + 1/12 + 1/18 = 1 (#2) • 1/2 + 1/5 + 1/6 + 1/12 + 1/20 = 1 (#3) • 1/3 + 1/4 + 1/5 + 1/6 + 1/20 = 1 (#4) • 1/2 + 1/4 + 1/8 + 1/12 + 1/24 = 1 (#5) • 1/2 + 1/3 + 1/12 + 1/21 + 1/28 = 1 (#6) • 1/2 + 1/4 + 1/6 + 1/21 + 1/28 = 1 (#7) • 1/2 + 1/4 + 1/7 + 1/14 + 1/28 = 1 (#8) • 1/2 + 1/3 + 1/12 + 1/20 + 1/30 = 1 (#9) • 1/2 + 1/4 + 1/6 + 1/20 + 1/30 = 1 (#10) • 1/2 + 1/5 + 1/6 + 1/10 + 1/30 = 1 (#11) • 1/2 + 1/3 + 1/11 + 1/22 + 1/33 = 1 (#12) • 1/2 + 1/3 + 1/14 + 1/15 + 1/35 = 1 (#13) • 1/2 + 1/3 + 1/12 + 1/18 + 1/36 = 1 (#14) • 1/2 + 1/4 + 1/6 + 1/18 + 1/36 = 1 (#15) • 1/2 + 1/3 + 1/10 + 1/24 + 1/40 = 1 (#16) • 1/2 + 1/4 + 1/8 + 1/10 + 1/40 = 1 (#17) • 1/2 + 1/4 + 1/7 + 1/12 + 1/42 = 1 (#18) • 1/2 + 1/3 + 1/9 + 1/30 + 1/45 = 1 (#19) • 1/2 + 1/4 + 1/5 + 1/36 + 1/45 = 1 (#20) • 1/2 + 1/5 + 1/6 + 1/9 + 1/45 = 1 (#21) • 1/2 + 1/3 + 1/12 + 1/16 + 1/48 = 1 (#22) • 1/2 + 1/4 + 1/6 + 1/16 + 1/48 = 1 (#23) • 1/2 + 1/3 + 1/9 + 1/27 + 1/54 = 1 (#24) • 1/2 + 1/3 + 1/8 + 1/42 + 1/56 = 1 (#25) • 1/2 + 1/3 + 1/8 + 1/40 + 1/60 = 1 (#26) • 1/2 + 1/3 + 1/10 + 1/20 + 1/60 = 1 (#27) • 1/2 + 1/3 + 1/12 + 1/15 + 1/60 = 1 (#28) • 1/2 + 1/4 + 1/5 + 1/30 + 1/60 = 1 (#29) • 1/2 + 1/4 + 1/6 + 1/15 + 1/60 = 1 (#30) • 1/2 + 1/4 + 1/5 + 1/28 + 1/70 = 1 (#31) • 1/2 + 1/3 + 1/8 + 1/36 + 1/72 = 1 (#32) • 1/2 + 1/3 + 1/9 + 1/24 + 1/72 = 1 (#33) • 1/2 + 1/4 + 1/8 + 1/9 + 1/72 = 1 (#34) • 1/2 + 1/3 + 1/12 + 1/14 + 1/84 = 1 (#35) • 1/2 + 1/4 + 1/6 + 1/14 + 1/84 = 1 (#36) • 1/2 + 1/3 + 1/8 + 1/33 + 1/88 = 1 (#37) • 1/2 + 1/3 + 1/10 + 1/18 + 1/90 = 1 (#38) • 1/2 + 1/3 + 1/7 + 1/78 + 1/91 = 1 (#39) • 1/2 + 1/3 + 1/8 + 1/32 + 1/96 = 1 (#40) • 1/2 + 1/3 + 1/9 + 1/22 + 1/99 = 1 (#41) • 1/2 + 1/4 + 1/5 + 1/25 + 1/100 = 1 (#42) • 1/2 + 1/3 + 1/7 + 1/70 + 1/105 = 1 (#43) • 1/2 + 1/3 + 1/11 + 1/15 + 1/110 = 1 (#44) • 1/2 + 1/3 + 1/8 + 1/30 + 1/120 = 1 (#45) • 1/2 + 1/4 + 1/5 + 1/24 + 1/120 = 1 (#46) • 1/2 + 1/5 + 1/6 + 1/8 + 1/120 = 1 (#47) • 1/2 + 1/3 + 1/7 + 1/63 + 1/126 = 1 (#48) • 1/2 + 1/3 + 1/9 + 1/21 + 1/126 = 1 (#49) • 1/2 + 1/3 + 1/7 + 1/60 + 1/140 = 1 (#50) • 1/2 + 1/4 + 1/7 + 1/10 + 1/140 = 1 (#51) • 1/2 + 1/3 + 1/12 + 1/13 + 1/156 = 1 (#52) • 1/2 + 1/4 + 1/6 + 1/13 + 1/156 = 1 (#53) • 1/2 + 1/3 + 1/7 + 1/56 + 1/168 = 1 (#54) • 1/2 + 1/3 + 1/8 + 1/28 + 1/168 = 1 (#55) • 1/2 + 1/3 + 1/9 + 1/20 + 1/180 = 1 (#56) • 1/2 + 1/3 + 1/7 + 1/54 + 1/189 = 1 (#57) • 1/2 + 1/3 + 1/8 + 1/27 + 1/216 = 1 (#58) • 1/2 + 1/4 + 1/5 + 1/22 + 1/220 = 1 (#59) • 1/2 + 1/3 + 1/11 + 1/14 + 1/231 = 1 (#60) • 1/2 + 1/3 + 1/7 + 1/51 + 1/238 = 1 (#61) • 1/2 + 1/3 + 1/10 + 1/16 + 1/240 = 1 (#62) • 1/2 + 1/3 + 1/7 + 1/49 + 1/294 = 1 (#63) • 1/2 + 1/3 + 1/8 + 1/26 + 1/312 = 1 (#64) • 1/2 + 1/3 + 1/7 + 1/48 + 1/336 = 1 (#65) • 1/2 + 1/3 + 1/9 + 1/19 + 1/342 = 1 (#66) • 1/2 + 1/4 + 1/5 + 1/21 + 1/420 = 1 (#67) • 1/2 + 1/3 + 1/7 + 1/46 + 1/483 = 1 (#68) • 1/2 + 1/3 + 1/8 + 1/25 + 1/600 = 1 (#69) • 1/2 + 1/3 + 1/7 + 1/45 + 1/630 = 1 (#70) • 1/2 + 1/3 + 1/7 + 1/44 + 1/924 = 1 (#71) • 1/2 + 1/3 + 1/7 + 1/43 + 1/1806 = 1 (#72)
All the sums start with 1/2 except for one:
• 1/2 + 1/5 + 1/6 + 1/12 + 1/20 = 1 (#3) • 1/3 + 1/4 + 1/5 + 1/6 + 1/20 = 1 (#4)
Here are the solutions in another format:
(2,4,10,12,15), (2,4,9,12,18), (2,5,6,12,20), (3,4,5,6,20), (2,4,8,12,24), (2,3,12,21,28), (2,4,6,21,28), (2,4,7,14,28), (2,3,12,20,30), (2,4,6,20,30), (2,5,6,10,30), (2,3,11,22,33), (2,3,14,15,35), (2,3,12,18,36), (2,4,6,18,36), (2,3,10,24,40), (2,4,8,10,40), (2,4,7,12,42), (2,3,9,30,45), (2,4,5,36,45), (2,5,6,9,45), (2,3,12,16,48), (2,4,6,16,48), (2,3,9,27,54), (2,3,8,42,56), (2,3,8,40,60), (2,3,10,20,60), (2,3,12,15,60), (2,4,5,30,60), (2,4,6,15,60), (2,4,5,28,70), (2,3,8,36,72), (2,3,9,24,72), (2,4,8,9,72), (2,3,12,14,84), (2,4,6,14,84), (2,3,8,33,88), (2,3,10,18,90), (2,3,7,78,91), (2,3,8,32,96), (2,3,9,22,99), (2,4,5,25,100), (2,3,7,70,105), (2,3,11,15,110), (2,3,8,30,120), (2,4,5,24,120), (2,5,6,8,120), (2,3,7,63,126), (2,3,9,21,126), (2,3,7,60,140), (2,4,7,10,140), (2,3,12,13,156), (2,4,6,13,156), (2,3,7,56,168), (2,3,8,28,168), (2,3,9,20,180), (2,3,7,54,189), (2,3,8,27,216), (2,4,5,22,220), (2,3,11,14,231), (2,3,7,51,238), (2,3,10,16,240), (2,3,7,49,294), (2,3,8,26,312), (2,3,7,48,336), (2,3,9,19,342), (2,4,5,21,420), (2,3,7,46,483), (2,3,8,25,600), (2,3,7,45,630), (2,3,7,44,924), (2,3,7,43,1806)

Note

Strictly speaking, there are two solutions for n = 2 in genuine Egyptian fractions, because 1/3 + 2/3 = 1 and 1/4 + 3/4 = 1. As noted above, 2/3 and 3/4 were permitted as fractions in ancient Egypt.

Overlord In Terms of Core Issues Around Maximal Engagement with Key Notions of the Über-Feral

მეფე (2) • მსოფლიოს (3) • მათემატიკა (5) •

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