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A031585 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 87. 0
7571, 7583, 7587, 7591, 7603, 7607, 7611, 7619, 7639, 7643, 7667, 7679, 7687, 7691, 7699, 7703, 7723, 7727, 7731, 7751, 7759, 7771, 7803, 7811, 7823, 7827, 7831, 7867, 7879, 7883, 7899, 7903, 7907, 7919, 30280, 30304, 30336, 30368, 30376, 30400, 30432 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..41.

MATHEMATICA

cf87Q[n_]:=Module[{s=Sqrt[n], cf, len}, cf=If[IntegerQ[s], {1}, ContinuedFraction[ s][[2]]]; len=Length[cf]; EvenQ[len]&&cf[[len/2]] == 87]; Select[Range[31000], cf87Q] (* Harvey P. Dale, Jan 24 2019 *)

CROSSREFS

Sequence in context: A206173 A205427 A252163 * A031765 A205740 A206672

Adjacent sequences:  A031582 A031583 A031584 * A031586 A031587 A031588

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified January 18 13:38 EST 2022. Contains 350455 sequences. (Running on oeis4.)