A031511 - OEIS

%I #14 Jul 22 2022 11:24:06

%S 171,179,187,191,199,211,223,680,704,768,800,808,832,864,872,896,1527,

%T 1587,1671,1815,1875,1959,2019,2712,2776,2824,3032,3208,3288,3352,

%U 3464,3544,3592,4235,4315,4415,4435,4535,4615,4735,4815,4835,4915,5035,5135

%N 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 13.

%t ct13Q[n_]:=Module[{s=Sqrt[n],cf},cf=If[IntegerQ[s],{1,1}, ContinuedFraction[ s][[2]]];EvenQ[Length[cf]]&&cf[[(Length[cf])/2]] == 13]; Select[Range[5200],ct13Q] (* _Harvey P. Dale_, Sep 04 2018 *)

%K nonn

%O 1,1

%A _David W. Wilson_

%E Definition edited by _Harvey P. Dale_, Jul 16 2022