## Thue-Morse Sequence

The Integer Sequence (also called the Morse-Thue Sequence) (1)

(Sloane's A010060) which arises in the Thue-Morse Constant. It can be generated from the Substitution Map   (2)   (3)

starting with 0 as follows: (4)

Writing the sequence as a Power Series over the Galois Field GF(2), (5)

then satisfies the quadratic equation (6)

This equation has two solutions, and , where is the complement of , i.e., (7)

which is consistent with the formula for the sum of the roots of a quadratic. The equality (6) can be demonstrated as follows. Let ( ...) be a shorthand for the Power series (8)

so is (0110100110010110...). To get , simply use the rule for squaring Power Series over GF(2) (9)

which extends to the simple rule for squaring a Power Series (10)

i.e., space the series out by a factor of 2, (0 1 1 0 1 0 0 1 ...), and insert zeros in the Odd places to get (11)

Then multiply by (which just adds a zero at the front) to get (12)

Adding to gives (13)

This is the first term of the quadratic equation, which is the Thue-Morse sequence with each term doubled up. The next term is , so we have   (14)   (15)

The sum is the above two sequences XORed together (there are no Carries because we're working over GF(2)), giving (16)

We therefore have (17)

The Thue-Morse sequence is an example of a cube-free sequence on two symbols (Morse and Hedlund 1944), i.e., it contains no substrings of the form , where is any Word. For example, it does not contain the Words 000, 010101 or 010010010. In fact, the following stronger statement is true: the Thue-Morse sequence does not contain any substrings of the form , where is the first symbol of . We can obtain a Squarefree sequence on three symbols by doing the following: take the Thue-Morse sequence 0110100110010110... and look at the sequence of Words of length 2 that appear: 01 11 10 01 10 00 01 11 10 .... Replace 01 by 0, 10 by 1, 00 by 2 and 11 by 2 to get the following: 021012021.... Then this Sequence is Squarefree (Morse and Hedlund 1944).

The Thue-Morse sequence has important connections with the Gray Code. Kindermann generates fractal music using the Self-Similarity of the Thue-Morse sequence.

References

Kindermann, L. MusiNum--The Music in the Numbers.'' http://www.forwiss.uni-erlangen.de/~kinderma/musinum/.

Morse, M. and Hedlund, G. A. Unending Chess, Symbolic Dynamics, and a Problem in Semigroups.'' Duke Math. J. 11, 1-7, 1944.

Schroeder, M. R. Fractals, Chaos, and Power Laws: Minutes from an Infinite Paradise. New York: W. H. Freeman, 1991.

Sloane, N. J. A. Sequence A010060 in The On-Line Version of the Encyclopedia of Integer Sequences.'' http://www.research.att.com/~njas/sequences/eisonline.html.