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 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. 0
 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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified May 24 16:32 EDT 2024. Contains 372781 sequences. (Running on oeis4.)