A031523 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 25.

627, 631, 643, 647, 659, 679, 683, 691, 699, 719, 727, 2504, 2536, 2560, 2656, 2688, 2720, 2752, 2784, 2792, 2816, 2912, 5631, 5703, 5835, 5847, 5919, 5991, 6051, 6207, 6339, 6351, 6423, 6495, 6555, 10008, 10072, 10264, 10328, 10376, 10456, 10648

Offset

1,1

Comments

"Central term" means the term at 1/2 the length of the period. - Harvey P. Dale, Dec 27 2011

Links

Table of n, a(n) for n=1..42.

Mathematica

cfevQ[n_]:=Module[{sr=Sqrt[n], cf, tr, len}, cf=ContinuedFraction[sr]; tr = !IntegerQ[sr]; len=If[tr, Length[cf[[2]]], 1]; tr&&EvenQ[len]&&cf[[2]][[len/2]]==25]; Select[Range[11000], cfevQ2] (* Harvey P. Dale, Dec 27 2011 *)

Crossrefs

Sequence in context: A031728 A031638 A098262 * A345523 A345777 A129974

Adjacent sequences: A031520 A031521 A031522 * A031524 A031525 A031526

Keyword

nonn

Author

David W. Wilson

Status

approved