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%I #15 Aug 19 2021 23:30:19
%S 809,2581,2785,2890,4217,5353,6949,7114,7621,9698,9893,10289,11177,
%T 11597,12461,15325,15817,17429,18218,18754,19298,19946,20506,20789,
%U 21361,22234,23833,25693,27085,27737,29065,29741,30202,30425,30545,30890,31237,32290
%N 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.
%t 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 *)
%K nonn
%O 1,1
%A _David W. Wilson_
%E Corrected and extended by _Harvey P. Dale_, Mar 18 2018
%E Definitions of A031599-A031688 clarified by _N. J. A. Sloane_, Aug 17 2021
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