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A031509
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Numbers k such that the continued fraction for sqrt(k) has even period and if the last term of the periodic part is deleted the central term is 11.
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2
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123, 127, 131, 139, 151, 163, 167, 488, 512, 520, 544, 608, 640, 672, 1095, 1167, 1383, 1455, 1515, 1944, 2008, 2136, 2264, 2456, 2648, 2696, 3035, 3115, 3215, 3235, 3415, 3515, 3635, 3715, 3735, 3835, 3935, 4115, 4135, 4215, 4368, 4944, 5496, 5943, 5971
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OFFSET
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1,1
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LINKS
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EXAMPLE
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The c.f. expansion of sqrt(127) is 11, [3, 1, 2, 2, 7, 11, 7, 2, 2, 1, 3, 22], [3, 1, 2, 2, 7, 11, 7, 2, 2, 1, 3, 22], ... If the 22 is deleted from the periodic part the central term is 11. - N. J. A. Sloane, Aug 17 2021
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MAPLE
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# Maple 2016 or later.
filter:= proc(n) uses NumberTheory; local R;
if issqr(n) then return false fi;
R:= Term(ContinuedFraction(sqrt(n)), periodic)[2];
nops(R)::even and R[nops(R)/2] = 11
end proc:
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MATHEMATICA
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okQ[k_] := Module[{c, lc}, If[IntegerQ[Sqrt[k]], False,
c = ContinuedFraction[Sqrt[k]]; lc = Length[c[[2]]];
EvenQ[lc] && c[[2, lc/2]] == 11]];
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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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