A031616 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 28.

5209, 16210, 16465, 16981, 24457, 35513, 45169, 46021, 58853, 59338, 60314, 72185, 74345, 75437, 92857, 106154, 107458, 108113, 110753, 123826, 124529, 125941, 127361, 128074, 128789, 131669, 151273, 168941, 170585, 172237, 173897

Offset: 1

Author

David W. Wilson

Keywords

• nonn

Crossrefs

Subsequence of A003814.

Programs

Mathematica

cf28Q[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]] == 28]; Rest[Select[Range[174000],cf28Q]] (* Harvey P. Dale, Jun 24 2020 *)

Extensions

First term 785 removed by Georg Fischer, Jun 16 2019