# Magistra Rules the Waves

One of my favourite integer sequences has the simple formula n(i) = n(i-1) + digitsum(n(i-1)). If it’s seeded with 1, its first few terms go like this:

n(1) = 1
n(2) = n(1) + digitsum(n(1)) = 1 + digitsum(1) = 2
n(3) = 2 + digitsum(2) = 4
n(4) = 4 + digitsum(4) = 8
n(5) = 8 + digitsum(8) = 16
n(6) = 16 + digitsum(16) = 16 + 1+6 = 16 + 7 = 23
n(7) = 23 + digitsum(23) = 23 + 2+3 = 23 + 5 = 28
n(8) = 28 + digitsum(28) = 28 + 2+8 = 28 + 10 = 38

As a sequence, it looks like this:

1, 2, 4, 8, 16, 23, 28, 38, 49, 62, 70, 77, 91, 101, 103, 107, 115, 122, 127, 137, 148, 161, 169, 185, 199, 218, 229, 242, 250, 257, 271, 281, 292, 305, 313, 320, 325, 335, 346, 359, 376, 392, 406, 416, 427, 440, 448, 464, 478, 497, 517, 530, 538, 554, 568, 587, 607, 620, 628, 644, 658, 677, 697, 719, 736, 752, 766, 785, 805, 818, 835, 851, 865, 884, 904, 917, 934, 950, 964, 983, 1003…

Given a number at random, is there a quick way to say whether it appears in the sequence seeded with 1? Not that I know, with one exception. If the number is divisible by 3, it doesn’t appear, at least in base 10. In base 2, that rule doesn’t apply:

n(1) = 1
n(2) = 1 + digitsum(1) = 10 = 1 + 1 = 2
n(3) = 10 + digitsum(10) = 10 + 1 = 11 = 2 + 1 = 3
n(4) = 11 + digitsum(11) = 11 + 1+1 = 101 = 3 + 2 = 5
n(5) = 101 + digitsum(101) = 101 + 1+0+1 = 111 = 5 + 2 = 7
n(6) = 111 + digitsum(111) = 111 + 11 = 1010 = 7 + 3 = 10
n(7) = 1010 + digitsum(1010) = 1010 + 10 = 1100 = 10 + 2 = 12
n(8) = 1100 + digitsum(1100) = 1100 + 10 = 1110 = 12 + 2 = 14

1, 2, 3, 5, 7, 10, 12, 14, 17, 19, 22, 25, 28, 31, 36, 38, 41, 44, 47, 52, 55, 60, 64, 65, 67, 70, 73, 76, 79, 84, 87, 92, 96, 98, 101, 105, 109, 114, 118, 123, 129, 131, 134, 137, 140, 143, 148, 151, 156, 160, 162, 165, 169, 173, 178, 182, 187, 193, 196, 199, 204, 208, 211, 216, 220, 225, 229, 234, 239, 246, 252, 258, 260, 262, 265, 268, 271, 276, 279, 284, 288, 290, 293, 297, 301, 306, 310, 315, 321, 324, 327, 332, 336, 339, 344, 348, 353, 357, 362, 367, 374…

What patterns are there in these sequences? It’s easier to check when they’re represented graphically, so I converted them into patterns à la the Ulam spiral, where n is represented as a dot on a spiral of integers. This is the spiral for base 10:

And these are the spirals for bases 2 and 3: Base 2 Base 3

These sequences look fairly random to me: there are no obvious patterns in the jumps from n(i) to n(i+1), i.e. in the values for digitsum(n(i)). Now try the spirals for bases 9 and 33: Base 9 Base 33

Patterns have appeared: there is some regularity in the jumps. You can see these regularities more clearly if you represent digitsum(n(i)) as a graph, with n(i) on the x axis and digitsum(n(i)) on the y axis. If the graph starts with n(i) = 1 on the lower left and proceeds left-right, left-right up the screen, it looks like this in base 10: Base 10 (click to enlarge)

Here are bases 2 and 3: Base 2 Base 3

The jumps seem fairly random. Now try bases 9, 13, 16, 17, 25, 33 and 49: Base 9 Base 13 Base 16 Base 17 Base 25 Base 33 Base 49

In some bases, the formula n(i) = n(i-1) + digitsum(n(i-1)) generates mild randomness. In others, it generates strong regularity, like waves rolling ashore under a steady wind. I don’t understand why, but regularity seems to occur in bases that are one more than a power of 2 and also in some bases that are primes or squares.

Elsewhere other-posted:

# Moto-Motto

Poem XLIII of Housman’s More Poems (1936) runs like this:

I wake from dreams and turning
My vision on the height
I scan the beacons burning

Inflaming heaven they flare;
They sign with conflagration
The empty moors of air.

The signal-fires of warning
They blaze, but none regard;
And on through night to morning
The world runs ruinward. (MP, XLIII)

In his commentary on the poem, the Housman scholar Archie Burnett traces a parallel with these lines from Lucretius: …multosque per annos | sustenata ruet moles et machina mundi – “…and the mass and fabric of the world, upheld through many years, shall crash into ruins” (De Rerum Natura, V 95-6).

I like the phrase moles et machina mundi, “mass and fabric of the world”, but I didn’t understand the translation fully. I investigated and discovered that the Latin word machina, though taken from Doric Greek μαχανα, makhana, “mechanical device”,* developed an additional meaning of “frame” or “body”. So Latin has deus ex machina, “god from the machine”, with one meaning, and machina mundi, “fabric of the world”, with another.

This seems to make machina a good word to expand the motto of this bijou bloguette. At the moment, the motto is this:

• Mathematica (v) • Magistra (iij) • Mundi (ij) •

That means “Mathematics is Mistress of the World”. Now try this:

• Mathematica (v) • Machina (iij) • Mundi (ij) •

The syllabification doesn’t change, but now I assume that the central word is pleasingly ambiguous and the motto means variously “Mathematics is Mechanism of the World”, the “Fabric of the World”, the “Engine of the World”, the “Body of the World”, and so on.

In addition, all the letters of Machina are found in Mathematica and Mundi, so the words on left and right almost act as a matrix, generating what appears between them.

There are further possibilities, blending magistra and machina:

• Mathematica (v) • Machistra (iij) • Mundi (ij) •

• Mathematica (v) • Magina (iij) • Mundi (ij) •

*In Attic Greek, it’s μηχανη, mēkhanē, whence “mechanical”, etc.

# On the M3!

6 = 2 x 3. And 6 = 1 + 2 + 3. But 6 also equals 3!. That is, 6 = 3 x 2 x 1, or factorial three. If you have three different items, you can arrange them in six different ways. There are three posibilities for the first item, two for the second and one for the third.

You can illustrate this linguistically. All languages are governed by mathematics, but maths manifests itself in different ways. Emphasis is an important part of language, for example, but there are different ways to achieve it. English usually does it with stress or by adding an emphatic word. Other languages can do it by varying the order of words. Latin, for example:

• Mathematica Magistra Mundi
— Mathematics is Mistress of the World.
• Mathematica Mundi Magistra
— Mathematics of the World is Mistress.
• Magistra Mathematica Mundi
— Mistress is Mathematics of the World
• Magistra Mundi Mathematica
— Mistress of the World is Mathematics.
• Mundi Mathematica Magistra
— Of the World Mathematics is Mistress.
• Mundi Magistra Mathematica
— Of the World the Mistress is Mathematics.

Elsewhere other-posted:

Mathematica Magistra Mundi — more on the motto
Moto-Motto — a variant on the motto

# He Say, He Sigh, He Sow #19

“The study of mathematics is the indispensable basis for all intellectual and spiritual progress.” — F.M. Cornford (1874-1943) quoted in The Sacred in Music (see also Pythagoreanism).

# They Say, They Sigh, They Sow #3

“Exchange rate behaves like particles in a molecular fluid” — ScienceDaily, 13/iii/2014.

# Poulet’s Propeller

The Penguin Dictionary of Curious and Interesting Numbers (1986) is one of my favourite books. It’s a fascinating mixture of math, mathecdote and math-joke:

2·618 0333…

The square of φ, the golden ratio, and the only positive number such that √n = n-1. (pg. 45)

6

Kepler discussed the 6-fold symmetry of snowflakes, and attempted to explain it by considering the close packing of spheres in a hexagonal array. (pg. 69)

39

This appears to be the first uninteresting number, which of course makes it an especially interesting number, because it is the smallest number to have the property of being uninteresting.

It is therefore also the first number to be simultaneously interesting and uninteresting. (pg. 120)

David Wells, who wrote the Dictionary, “had the rare distinction of being a Cambridge scholar in mathematics and failing his degree”. He must be the mathematical equivalent of the astronomer Patrick Moore: a popularizer responsible for opening many minds and inspiring many careers. He’s also written books on geometry and mathematical puzzles. But not everyone appreciates his efforts. This is a sideswipe in a review of William Hartston’s The Book of Numbers:

Thankfully, this book is more concerned with facts than mathematics. Anyone wanting to learn more about [π] or the Fibonacci sequence should turn to the Penguin Dictionary of Curious and Interesting Numbers, a volume which none but propeller-heads will find either curious or interesting. (Review in The Independent, 18th December 1997)