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A258703
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a(n) = floor(n/sqrt(2) - 1/2).
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1
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0, 0, 1, 2, 3, 3, 4, 5, 5, 6, 7, 7, 8, 9, 10, 10, 11, 12, 12, 13, 14, 15, 15, 16, 17, 17, 18, 19, 20, 20, 21, 22, 22, 23, 24, 24, 25, 26, 27, 27, 28, 29, 29, 30, 31, 32, 32, 33, 34, 34, 35, 36, 36, 37, 38, 39, 39, 40, 41, 41, 42, 43, 44, 44, 45, 46, 46, 47, 48, 48, 49, 50, 51, 51, 52, 53, 53
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OFFSET
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1,4
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COMMENTS
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By definition, (a(n)) is an inhomogeneous Beatty sequence. The associated Sturmian word is s(alpha, rho) = (floor((n + 1)*alpha + rho) - floor(n*alpha + rho), n= 0, 1, 2,...) = 1,0,1,1,1,0,1,1,0,1,1,..., with slope alpha = sqrt(2)/2, and intercept rho = -1/2.
Also, s(alpha, rho) = s(alpha,rho+1) - 1. Since 0 < alpha < 1 and 0 < rho +1 < 1, with algebraic conjugates
alpha* = -sqrt(2)/2, and (rho +1)* = 1/2,
Yasutomi's criterion gives that s(alpha, rho) is fixed point of a morphism.
The morphism can be found following the ideas of Chapter 2 in Lothaire's book and Section 4 of my paper "Substitution invariant Sturmian words and binary trees" (cf. A006340).
For a better fit with the literature we will determine the morphism that fixes the binary complement s(1-alpha, 1-(1+rho) ) = 0,1,0,0,0,1,0,0,1,0,0....
Let psi_1 and psi_8 be the elementary Sturmian morphisms given by
psi_1(0)=01 , psi_1(1)=0; psi_8(0)=01, psi(8)1=1.
Let psi := psi_1 psi_8. Then psi is given by
psi(0)=010 , psi(1)=0.
We see that psi fixes the Octanacci sequence A324772.
That psi is the right morphism can be proved by checking that (x,y) = (1-alpha, -rho) is fixed point of the composition T_1 T_8 of the fractional linear maps
T_1(x,y) = ((1-x)/(2-x), (1-y)/(2-x)),
T_8(x,y) = ((1/(2-x), y/(2-x))).
Conclusion, taking the binary complement of psi: the Sturmian word equals A104521. (End)
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LINKS
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FORMULA
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a(n) = floor(1/(exp(sqrt(2)/n)-1)) for all positive integers n [O'Bryant].
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MATHEMATICA
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PROG
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(Haskell)
a258703 = floor . (/ 2) . subtract 1 . (* sqrt 2) . fromIntegral
(PARI) vector(100, n, n--; floor(n/sqrt(2) - 1/2)) \\ G. C. Greubel, Sep 30 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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