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%I #15 Aug 18 2021 00:10:32
%S 41641,81274,133306,134770,199037,202621,278953,281069,283193,373546,
%T 479153,593261,733177,740041,869153,871018,878498,884129,1026794,
%U 1028821,1032881,1036949,1038986,1043066,1049201,1215133,1388525,1393241,1397965
%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 81.
%t cf81Q[n_]:=Module[{s=Sqrt[n],cf,len},cf=If[IntegerQ[s],{1,1}, ContinuedFraction[ s][[2]]];len=Length[cf];OddQ[len]&&cf[[(len+1)/2]] == 81]; Select[Range[14*10^5],cf81Q] (* _Harvey P. Dale_, Sep 15 2017 *)
%K nonn
%O 1,1
%A _David W. Wilson_
%E Corrected and extended by _Harvey P. Dale_, Sep 15 2017