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A308778
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Central element(s) in the period of the continued fraction expansion of sqrt(n), or 0 if no such element exists, or -1 if n is a square.
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2
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-1, -1, 0, 1, -1, 0, 2, 1, 1, -1, 0, 3, 2, 1, 2, 1, -1, 0, 4, 3, 2, 2, 4, 3, 1, -1, 0, 5, 2, 1, 2, 5, 1, 2, 4, 1, -1, 0, 6, 4, 3, 2, 2, 5, 2, 2, 6, 5, 1, -1, 0, 7, 2, 1, 6, 2, 2, 4, 1, 7, 2, 2, 6, 1, -1, 0, 8, 7, 4, 4, 2, 7, 2, 5, 1, 1, 4, 2, 4, 7, 1, -1, 0
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OFFSET
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0,7
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COMMENTS
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The continued fraction expansion of sqrt(n) is periodic (where n is no square), and the period splits in two halves which are mirrored around the center. With r = floor(sqrt(n)) the expansion takes one of the forms:
[r; i, j, k, ..., m, m, ..., k, j, i, 2*r] (odd period length) or
[r; i, j, k, ..., m, ..., k, j, i, 2*r] (even period length)
[r; 2*r] (empty symmetric part, for n = r^2 + 1)
This sequence lists the central element(s) m, or 0 for n = r^2 + 1, or -1 for n = r^2.
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LINKS
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EXAMPLE
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CF(sqrt(2906)) = [53;1,9,1,3,1,3,1,1,14,1,5,2,2,5,1,14,1,1,3,1,3,1,9,1,106], odd period, two central elements, a(2906) = 2.
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MAPLE
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f:= proc(n) local L, m;
if issqr(n) then return -1
elif issqr(n-1) then return 0
fi;
L:= numtheory:-cfrac(sqrt(n), periodic, quotients);
m:= nops(L[2]);
L[2][floor(m/2)]
end proc:
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MATHEMATICA
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Array[Which[IntegerQ@ Sqrt@ #, -1, IntegerQ@ Sqrt[# - 1], 0, True, #[[Floor[Length[#]/2]]] &@ Last@ ContinuedFraction@ Sqrt@ #] &, 83, 0] (* Michael De Vlieger, Jul 07 2019 *)
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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