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 A031601 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 13. 1
 409, 1181, 2258, 3562, 3802, 3925, 5305, 5897, 8269, 9829, 10834, 11041, 12389, 15737, 16745, 17785, 21757, 23029, 23633, 24554, 25178, 26869, 27194, 27521, 28181, 28514, 28849, 30554, 30901, 32413, 33853, 36073, 36185, 40841, 41362, 41765, 42170 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..150 MATHEMATICA cf13Q[n_]:=Module[{per=ContinuedFraction[Sqrt[n]][[2]]}, OddQ[Length[ per]]&&per[[Floor[Length[per]/2]+1]]==13]; nn=43000; With[{terms = Complement[Range[nn], Range[Floor[Sqrt[nn]]]^2]}, Select[terms, cf13Q]] (* Harvey P. Dale, Nov 23 2011 *) CROSSREFS Subsequence of A003814. Sequence in context: A321151 A052376 A201483 * A107625 A061333 A262636 Adjacent sequences: A031598 A031599 A031600 * A031602 A031603 A031604 KEYWORD nonn AUTHOR David W. Wilson EXTENSIONS Corrected by Harvey P. Dale, Nov 23 2011 STATUS approved

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Last modified July 19 16:24 EDT 2024. Contains 374410 sequences. (Running on oeis4.)