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A286665
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{0->01}-transform of the Pell word, A171588.
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4
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0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1
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OFFSET
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1
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COMMENTS
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Let psi_8 be the elementary Sturmian morphism given by psi_8(0)=01, psi_(8)1=1, and let x = A171588 be the Pell word. Then, by definition, (a(n)) = psi_8(x).
Now x is a Sturmian sequence x = s(alpha, rho) with slope alpha = 1-sqrt(2)/2 and intercept rho = alpha. This implies that (a(n)) is a Sturmian sequence with slope alpha' = 1/(2-alpha) = 2-sqrt(2), and intercept rho' = rho/(2-alpha) = 3-2*sqrt(2) (cf. Lothaire Lemma 2.2.18).
Since the algebraic conjugate of rho' is equal to 3+2*sqrt(2), which is larger than the algebraic conjugate of alpha', (a(n)) is NOT a fixed point of a morphism, by Yasutomi's criterion. However, (a(n)) IS a fixed point of an automorphism sigma of the free group generated by 0 and 1. In fact sigma: 0-> 01010^{-1}, 1-> 01.
To see this, let pi be the Pell morphism given by pi(0)=001, pi(1)=0. Then pi(x) = x, and so psi_8(pi(x)) = psi_8(x) = a, implying that sigma := psi_8 pi psi_8^{-1} fixes a = (a(n)). One easily computes psi_8^{-1}: 0->01^{-1}, 1->1, which gives sigma.
(End)
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LINKS
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FORMULA
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a(n) = [n*alpha+rho]-[(n-1)*alpha+rho], where alpha = 2-sqrt(2), and rho = 3-2*sqrt(2). - Michel Dekking, Mar 11 2018
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EXAMPLE
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As a word, A171588 = 001001000100100010010010001001000..., and replacing each 0 by 01 gives 010110101101010110101101010110...
To see that (a(n)) is fixed by sigma, iterate sigma starting with 01:
sigma(01) = 01010^{-1}01 = 01011,
sigma^2(01) = 01010^{-1}0101010^{-1}0101 = 010110101101. (End)
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MATHEMATICA
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s = Nest[Flatten[# /. {0 -> {0, 0, 1}, 1 -> {0}}] &, {0}, 6] (* A171588 *)
w = StringJoin[Map[ToString, s]]
w1 = StringReplace[w, {"0" -> "01"}]
st = ToCharacterCode[w1] - 48 ; (* A286665 *)
p0 = Flatten[Position[st, 0]]; (* A286666 *)
p1 = Flatten[Position[st, 1]]; (* A286667 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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