A031521 - OEIS

%I #22 Jun 08 2023 13:39:15

%S 531,547,563,571,587,599,603,607,619,623,2120,2144,2152,2176,2216,

%T 2240,2344,2368,2432,2440,2496,4767,4839,4911,4983,5055,5127,5199,

%U 5271,5487,5547,5619,8472,8664,8728,8856,8984,9112,9304,9352,9368,9432,9496,9624

%N Numbers k such that the continued fraction for sqrt(k) has even period and if the last term of the periodic part is deleted the central term is 23.

%C The "central term" is the term that appears at 1/2 the length of the period of the continued fraction. - _Harvey P. Dale_, Feb 25 2012

%t cf2Q[n_]:=Module[{cf=ContinuedFraction[Sqrt[n]],len},If[Length[cf]==1, len=1,len=Length[cf[[2]]]];EvenQ[len]&&cf[[2,len/2]]==23]; Select[ Range[10000],cf2Q](* _Harvey P. Dale_, Feb 26 2012 *)

%K nonn

%O 1,1

%A _David W. Wilson_

%E Definition edited by _Harvey P. Dale_, Jun 08 2023