A031669 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 81.

41641, 81274, 133306, 134770, 199037, 202621, 278953, 281069, 283193, 373546, 479153, 593261, 733177, 740041, 869153, 871018, 878498, 884129, 1026794, 1028821, 1032881, 1036949, 1038986, 1043066, 1049201, 1215133, 1388525, 1393241, 1397965

Offset

1,1

Links

Table of n, a(n) for n=1..29.

Mathematica

cf81Q[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]] == 81]; Select[Range[14*10^5], cf81Q] (* Harvey P. Dale, Sep 15 2017 *)

Crossrefs

Sequence in context: A262801 A253681 A178800 * A028239 A248445 A339796

Adjacent sequences: A031666 A031667 A031668 * A031670 A031671 A031672

Keyword

nonn

Author

David W. Wilson

Extensions

Corrected and extended by Harvey P. Dale, Sep 15 2017

Status

approved