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 A076921 Smallest number such that the highest common factor of pair of successive terms follows the pattern 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, ..., i.e., b(2n-1) = b(2n) = n given by A004526. 1
 1, 1, 2, 4, 6, 9, 12, 16, 20, 25, 30, 36, 42, 49, 56, 64, 72, 81, 90, 100, 110, 121, 132, 144, 156, 169, 182, 196, 210, 225, 240, 256, 272, 289, 306, 324, 342, 361, 380, 400, 420, 441, 462, 484, 506, 529, 552, 576, 600, 625, 650, 676, 702, 729, 756, 784 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS (1) a(2n) = n^2, a(2n-1) = n(n+1) = twice the n-th triangular number. (2) Geometric mean of the successive squares interleaved between them. Essentially the same as A002620. LINKS G. C. Greubel, Table of n, a(n) for n = 1..5000 Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA Empirical: a(n+1) + a(n) = binomial(n+1,2), a(1) = a(2) = 1. - G. C. Greubel, Oct 29 2017 MATHEMATICA Join[{1}, LinearRecurrence[{2, 0, -2, 1}, {1, 2, 4, 6}, 50]] (* G. C. Greubel, Oct 29 2017 *) CROSSREFS Cf. A004526. Sequence in context: A086378 A088900 A083392 * A002620 A087811 A025699 Adjacent sequences:  A076918 A076919 A076920 * A076922 A076923 A076924 KEYWORD nonn,easy AUTHOR Amarnath Murthy, Oct 17 2002 EXTENSIONS More terms from Philippe Deléham, Jun 20 2005 STATUS approved

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Last modified September 24 08:31 EDT 2021. Contains 347623 sequences. (Running on oeis4.)