A036581 - OEIS

A036581

Ternary Thue-Morse sequence: closed under a->abc, b->ac, c->b.

0, 2, 1, 0, 1, 2, 0, 2, 1, 2, 0, 1, 0, 2, 1, 0, 1, 2, 0, 1, 0, 2, 1, 2, 0, 2, 1, 0, 1, 2, 0, 2, 1, 2, 0, 1, 0, 2, 1, 2, 0, 2, 1, 0, 1, 2, 0, 1, 0, 2, 1, 0, 1, 2, 0, 2, 1, 2, 0, 1, 0, 2, 1, 0, 1, 2, 0, 1, 0, 2, 1, 2, 0, 2, 1, 0, 1, 2, 0, 1, 0, 2, 1, 0, 1, 2, 0, 2, 1, 2, 0, 1, 0, 2, 1, 2, 0, 2, 1

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OFFSET

0,2

COMMENTS

This sequence and A108694 are squarefree (they do not contain any substring XX). - Bill Gosper, Jul 22 2005

Trajectory of 1 under the morphism 0 -> 021, 1 -> 2 & 2 -> 01. - Robert G. Wilson v, Apr 06 2008

I believe that this is the sequence Cummings refers to as the Morse-Hedlund sequence. It can be constructed by starting with the Thue-Morse binary sequence A010060, 0,1,1,0,1,0,0,1,1,0,0,1,0,1,1,0,1,0,0,1,0,1,1,0,..., reading successive pairs of digits: 01, 11, 10, 01, 10, 00, 01, 11, 10, 00, 01, ..., and mapping 01 to 0, 10 to 1, and both 00 and 11 to 2, getting 0,2,1,0,1,2,0,2,1,... - N. J. A. Sloane, Oct 17 2012

REFERENCES

L. J. Cummings, On the construction of Thue sequences, Proc. 9th S-E Conf. Combinatorics, Graph Theory and Computing, pp. 235-242. - From N. J. A. Sloane, Oct 17 2012

M. Lothaire, Combinatorics on Words. Addison-Wesley, Reading, MA, 1983, p. 26.

LINKS

R. J. Mathar, Table of n, a(n) for n = 0..10000

A. Cobham, Uniform Tag Sequences, Mathematical Systems Theory, 6 (1972), 164-192.

Marston Morse and Gustav A. Hedlund, Unending chess, symbolic dynamics and a problem in semigroups, Duke Math. J., Volume 11, Number 1 (1944), 1-7. - From N. J. A. Sloane, Oct 17 2012

Index entries for sequences that are fixed points of mappings

FORMULA

a(n) = A010060(n+1) - A010060(n) - 1 mod 3. - Bill Gosper, Jul 22 2005

MAPLE

A036581 := proc(n)

A010060(n+1)-A010060(n)-1 ;

% mod 3 ;

end proc: # R. J. Mathar, Oct 17 2012

MATHEMATICA

Nest[ # /. {0 -> {0, 2, 1}, 1 -> {2}, 2 -> {0, 1}} &, {0}, 7] // Flatten (* Robert G. Wilson v, Apr 06 2008 *)

a010060[n_]:=Mod[DigitCount[n, 2, 1], 2]; Table[Mod[a010060[n + 1] - a010060[n] - 1, 3], {n, 0, 100}] (* Indranil Ghosh, Apr 25 2017 *)

SubstitutionSystem[{0->{0, 2, 1}, 1->{2}, 2->{0, 1}}, {0}, {7}][[1]] (* Harvey P. Dale, Dec 26 2021 *)

PROG

(Haskell)

a036581 n = a036581_list !! n

a036581_list = zipWith (\u v -> if u /= v then 2 * u + v - 1 else 2)

a010060_list $ tail a010060_list

-- According to N. J. A. Sloane's comment from Oct 17 2012

-- Reinhard Zumkeller, Nov 30 2012

(Python)

def a010060(n): return bin(n)[2:].count("1")%2

def a(n): return (a010060(n + 1) - a010060(n) - 1)%3 # Indranil Ghosh, Apr 25 2017

CROSSREFS

Cf. A036583, A108694.

Sequence in context: A179286 A193690 A108964 * A369462 A135055 A265433

Adjacent sequences: A036578 A036579 A036580 * A036582 A036583 A036584

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Robert G. Wilson v, Apr 06 2008

STATUS

approved