A031513 - OEIS

%I #14 Jul 06 2024 15:55:06

%S 227,239,243,251,263,267,271,283,287,904,928,1000,1096,1120,1152,2031,

%T 2103,2175,2235,2247,2319,2391,2463,2523,2595,3608,3672,3736,3928,

%U 3992,4184,4232,4248,4376,4504,4568,4616,5635,5735,5815,5935,6015,6115,6135

%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 15.

%t epct15Q[n_]:=Module[{s=Sqrt[n],cf},cf=If[IntegerQ[s],{1}, ContinuedFraction[ s][[2]]];EvenQ[Length[cf]]&&cf[[Length[cf]/2]] ==15]; Select[Range[6200],epct15Q] (* _Harvey P. Dale_, May 30 2013 *)

%K nonn

%O 1,1

%A _David W. Wilson_