# The Mathematician as an Explorer

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#### 50 Years Ago: Drummers and Mathematics

Editor's note: This article originally appeared in the May 1961 issue of Scientific American. Sherman K. Stein is currently professor emeritus of mathematics at University of California, Davis. He is the author of several popular books on mathematics published after this article; his latest is "Survival Guide for Outsiders: How to Protect Yourself from Politicians, Experts and Other Insiders: (BookSurge Publishing, 2010).

The nature of mathematics is elucidated by one mathematician's account of how a memory word used by drummers in ancient India led him to the classic problem of the traveling salesman's route

Mathematics, like every branch of knowledge, is the product of the interplay between past and present, between accumulated knowledge and curiosity, between an autonomous structure and the tastes and needs of the time. What one age considers a pressing question, another may not ask at all. The pure mathematics of one era may be applied in another, perhaps centuries later.

A problem with which I recently tangled illustrates these aspects of the growth of knowledge, and its solution captures the adventurous flavor of mathematical research. Pushing into the unknown, the mathematician is an explorer who is likely to find what he did not seek and who cannot predict how others will use his discoveries.

This particular adventure began when the composer George Perle told me about an elaborate theory of rhythm that had been developed in India more than a thousand years ago. "While reading about this theory," he said, "I learned my one and only Sanskrit word: yamátárájahánsalagám." I asked him what it meant.

"It's just a nonsense word invented as a memory aid for Indian drummers."

"If a drummer can remember that, " I replied, "he can remember anything. "

"There is a lot in those ten syllables, " said Perle. "As you pronounce the word you sweep out all possible triplets of short and long beats. The first three syllables, ya má tá, have the rhythm short, long, long. The second through the fourth are má tá rá: long, long, long. Then you have tá rá já: long, long, short. "Next there are rá ja bhá: long, short, long. And so on." I wrote down the word and saw that what Perle said is true. Each successive triplet of syllables displays a different pattern, and the whole word displays all the possible patterns, giving each one once and only once. As a mathematician I was fascinated to find that such a sequence could exist.

That night I returned to the ancient word. To strip it of irrelevancies I replaced the syllables with digits, letting 0 stand for short beats and 1 for long. In this notation yamátárájahánsalagám became 0111010000l.

Staring at the simplified string for a while, I noticed a lovely thing. The first two digits are the same as the last two; so if I bent the string into a loop, it would look like a snake swallowing its own tail. That is, the last 01 could be placed over the first 01, so that the two pairs of digits would merge into a single pair. Instead of a line of 10 digits I now saw a circle of eight.

I could begin anywhere on this "memory wheel" and move around it in either direction, sweeping out triplets of 0's and l's. Starting at the top and reading clockwise, for example, gave 011, 111, 110, 101, 010, 100, 000, 00l.

The next thing that occurred to me, as it would automatically to any mathematician,was to generalize what I had found. Is there a "word" for listing all quadruplets of 0's and l's once and only once? For quintuplets? For groups of any size? And if so, does the snake always swallow its tail?

Before attacking the problem for quadruplets it seemed sensible to go back and look at couplets. Is there a word that lists each of the four couplets 00, 01, 10, 11 exactly once? Does it close up on itself? 'Writing down the sequence 0011, I saw that I already had the three couplets 00, 01, 11. Adding one 0 more, to form 00110, gave the final couplet: 10. I noted that the first and last digits are O's, so that the snake swallows its tail. The five-digit word could be bent into a four-digit wheel containing each couplet once and only once.

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1. 1. marktempeit 08:42 AM 5/4/11

0111010000l

should be

0111010001

(one 0 less and the last character being the number 1 instead of the letter 'l')

2. 2. thijs_t 10:53 AM 5/4/11

A linear feedback shift register (used for e.g. cryptography) uses characteristic polynomials of finite fields of size 2^n to generate such lists of 0's and 1's, containing all the 2^n-1 nonzero states. Adding a zero in the current longest sequence of zeros yields a complete list.

3. 3. Will_in_BC 11:56 AM 5/4/11

Nice article.

I think there is also a data compression angle as well. The encoding of the patterns takes the fewest bits possible with the ability to read off a unique pattern at each bit position.

4. 4. ashwinmudigonda 12:37 PM 5/4/11

What are the odds! I am an Indian, currently using a variant of the traveling salesman problem to solve and engineering problem. I would like to quickly point out that the mnemonic is not all garbage. Sanskrit, like Latin, is a language that allows the fusion of multiple roots to create a composite word. Though my Sanskrit is rusty, I see

ya - not sure
mátá - Mother
rája - king
hánsa - swan (Lufthansa!)
lagám - not sure

Not that it matters, but many chants and mantras in Sanskrit appear to be garbage, but serve other purposes like this one.

5. 5. pawtiger 02:32 PM 5/4/11

Is the author aware of the Karmarkar algorithm? This was produced by an Indian-born researcher in the early 1990s to solve traveling-salesman problems on supercomputers. I would be interested in the algorithm's relationship to memory wheels.

6. 6. desai 12:35 AM 5/6/11

Interesting article but don't know if it is relevant in terms of finding a solution to TSP. For one, it seems to assume a complete graph. Secondly, it applies only graphs with number of cities that are a power of 2.

7. 7. pbathavale 08:54 PM 5/7/11

Original Sanskrit word is "yamatarajabhansalgam"
*
i.e."bha" i/o "ba"
When we learn grammar of Sanskrit Poetics(Prosody),"Kaavyashaastra"our Guru used to cite this abbreviated form in order to "Remember the Metre of the Song/Verse in order that we can sing in the Perfect Lilting Tunes that is Desired."
All Sanskrit based languages including Marathi , my mother tongue in Maharashtra State use this Formula.
Mr.PRADEEP ATHAVALE . B.Sc.,PUNE CITY ,INDIA 410016 .

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