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Numbers k such that the continued fraction for sqrt(k) has period 42.
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%I #13 Jul 11 2021 07:23:51

%S 571,724,889,907,1081,1348,1477,1576,1619,1633,1636,1732,1786,1841,

%T 2073,2167,2253,2276,2339,2344,2433,2493,2518,2611,2819,2821,2896,

%U 2901,2939,3039,3070,3142,3154,3203,3284,3430,3439,3467,3513,3523,3546,3556,3573

%N Numbers k such that the continued fraction for sqrt(k) has period 42.

%t cf42Q[n_]:=Module[{s=Sqrt[n]},If[IntegerQ[s],1,Length[ ContinuedFraction[ s][[2]]]]==42]; Select[Range[4000],cf42Q] (* _Harvey P. Dale_, Sep 24 2019 *)

%K nonn

%O 1,1

%A _David W. Wilson_