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%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
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