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 A031407 Numbers k such that the continued fraction for sqrt(k) has even period 2*m and the m-th term is 4. 1
 18, 22, 34, 39, 57, 68, 69, 76, 78, 92, 105, 108, 116, 124, 140, 150, 155, 174, 186, 203, 205, 217, 220, 259, 264, 266, 282, 294, 301, 308, 318, 329, 333, 369, 371, 376, 378, 406, 410, 413, 423, 434, 450, 456, 477, 490, 495, 504, 517, 522, 549, 550, 558 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 MATHEMATICA n = 1; t = {}; While[Length[t] < 50, n++; If[! IntegerQ[Sqrt[n]], c = ContinuedFraction[Sqrt[n]]; len = Length[c[[2]]]; If[EvenQ[len] && c[[2, len/2]] == 4, AppendTo[t, n]]]]; t (* T. D. Noe, Apr 04 2014 *) CROSSREFS Cf. A031404-A031423. Sequence in context: A096282 A250737 A290172 * A267826 A339473 A002505 Adjacent sequences:  A031404 A031405 A031406 * A031408 A031409 A031410 KEYWORD nonn AUTHOR STATUS approved

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Last modified July 24 21:47 EDT 2021. Contains 346273 sequences. (Running on oeis4.)