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 A031521 Numbers n such that continued fraction for sqrt(n) has even period and central term 23. 0
 531, 547, 563, 571, 587, 599, 603, 607, 619, 623, 2120, 2144, 2152, 2176, 2216, 2240, 2344, 2368, 2432, 2440, 2496, 4767, 4839, 4911, 4983, 5055, 5127, 5199, 5271, 5487, 5547, 5619, 8472, 8664, 8728, 8856, 8984, 9112, 9304, 9352, 9368, 9432, 9496, 9624 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The "central term" is the term that appears at 1/2 the length of the period of the continued fraction. [From Harvey P. Dale, Feb 25 2012] LINKS MATHEMATICA cf2Q[n_]:=Module[{cf=ContinuedFraction[Sqrt[n]], len}, If[Length[cf]==1, len=1, len=Length[cf[[2]]]]; EvenQ[len]&&cf[[2, len/2]]==23]; Select[ Range[10000], cf2Q](* From Harvey P. Dale, Feb 26 2012 *) CROSSREFS Sequence in context: A098259 A174761 A031967 * A156772 A031701 A158367 Adjacent sequences:  A031518 A031519 A031520 * A031522 A031523 A031524 KEYWORD nonn AUTHOR STATUS approved

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