A031672 - OEIS

%I #12 Aug 18 2021 00:10:32

%S 1765,143722,144481,215417,405994,511514,514378,515813,518689,781897,

%T 796105,941429,947258,951154,1105226,1122106,1126346,1304893,1494125,

%U 1499017,1508825,1513741,1711825,1714442,1730186,1732817,1927633,1933189,1941538

%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 84.

%t cf84Q[n_]:=Module[{s=Sqrt[n],c,len},c=If[IntegerQ[s],{0,0}, ContinuedFraction[ s][[2]]];len=Length[c];OddQ[len]&&c[[(len+1)/2]] == 84]; Select[Range[2*10^6],cf84Q] (* _Harvey P. Dale_, Jul 31 2016 *)

%K nonn

%O 1,1

%A _David W. Wilson_

%E Corrected and extended by _Harvey P. Dale_, Jul 31 2016