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%I #7 May 19 2024 14:26:49
%S 2575,8383,9919,11659,11771,13375,14999,16891,17875,18067,18232,18415,
%T 20087,22355,24568,27083,27783,29727,34867,35875,38056,40067,41987,
%U 42319,44063,44455,46343,48168,48216,50856,52219,52507,52871,53387,53447,54487
%N Period of continued fraction for sqrt(n) contains exactly 21 ones.
%t cf21Q[n_]:=With[{s=Sqrt[n]},If[IntegerQ[s],1,Count[ContinuedFraction[s][[2]],1]]==21]; Select[ Range[55000],cf21Q] (* _Harvey P. Dale_, May 19 2024 *)
%K nonn
%O 1,1
%A _David W. Wilson_