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 A031588 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 90. 0
 8102, 8114, 8126, 8158, 8166, 8182, 8194, 8198, 8222, 8254, 8258, 8262, 8278, 8302, 8306, 8318, 8338, 8342, 8402, 8406, 8414, 8422, 8434, 8438, 8454, 8462, 18228, 18237, 18273, 18291, 18294, 18366, 18372, 18393, 18399, 18417, 18429, 18453, 18489, 18507 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS MATHEMATICA cf90Q[n_]:=Module[{s=Sqrt[n], cf, len}, cf=If[IntegerQ[s], {1}, ContinuedFraction[ s][[2]]]; len=Length[cf]; EvenQ[len]&&cf[[len/2]]==90]; Select[Range[20000], cf90Q] (* Harvey P. Dale, Mar 30 2020 *) CROSSREFS Sequence in context: A172775 A172843 A258541 * A145657 A224727 A163045 Adjacent sequences: A031585 A031586 A031587 * A031589 A031590 A031591 KEYWORD nonn AUTHOR STATUS approved

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Last modified March 25 23:47 EDT 2023. Contains 361529 sequences. (Running on oeis4.)