A031599 - OEIS

A031599

Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted the two central terms are both 11.

809, 2581, 2785, 2890, 4217, 5353, 6949, 7114, 7621, 9698, 9893, 10289, 11177, 11597, 12461, 15325, 15817, 17429, 18218, 18754, 19298, 19946, 20506, 20789, 21361, 22234, 23833, 25693, 27085, 27737, 29065, 29741, 30202, 30425, 30545, 30890, 31237, 32290

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..38.

MATHEMATICA

cf11Q[n_]:=Module[{s=Sqrt[n], len, cf}, cf=If[IntegerQ[s], {2, 2}, ContinuedFraction[ s][[2]]]; len=Length[cf]; OddQ[len]&&cf[[(len+1)/2]] == 11]; Select[Range[32000], cf11Q] (* Harvey P. Dale, Mar 18 2018 *)

CROSSREFS

Sequence in context: A160034 A273811 A252028 * A265984 A332848 A160561

Adjacent sequences: A031596 A031597 A031598 * A031600 A031601 A031602

KEYWORD

nonn

AUTHOR

David W. Wilson

EXTENSIONS

Corrected and extended by Harvey P. Dale, Mar 18 2018

Definitions of A031599-A031688 clarified by N. J. A. Sloane, Aug 17 2021

STATUS

approved