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 A236257 a(n) = 2*n^2 - 7*n + 9. 2
 9, 4, 3, 6, 13, 24, 39, 58, 81, 108, 139, 174, 213, 256, 303, 354, 409, 468, 531, 598, 669, 744, 823, 906, 993, 1084, 1179, 1278, 1381, 1488, 1599, 1714, 1833, 1956, 2083, 2214, 2349, 2488, 2631, 2778, 2929, 3084, 3243, 3406, 3573, 3744, 3919, 4098, 4281 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS If zero polygonal numbers are ignored, then for n>=3, the a(n)-th n-gonal number is a sum of the (a(n)-1)-th n-gonal number and the (2*n-3)-th n-gonal number. LINKS Michael De Vlieger, Table of n, a(n) for n = 0..10000 W. Burrows, C. Tuffley, Maximising common fixtures in a round robin tournament with two divisions, arXiv preprint arXiv:1502.06664 [math.CO], 2015. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 3*a(n-1)-3*a(n-2)+a(n-3). G.f.: -(18*x^2-23*x+9) / (x-1)^3. - Colin Barker, Jan 21 2014 EXAMPLE a(7)=58. This means that the 58th heptagonal number 8323 (cf. A000566) is a sum of two heptagonal numbers. We have 8323 = 8037 + 286 with indices in A000566 58,57,11. MATHEMATICA Table[2 n^2 - 7 n + 9, {n, 0, 48}] (* Michael De Vlieger, Apr 19 2015 *) LinearRecurrence[{3, -3, 1}, {9, 4, 3}, 50] (* Harvey P. Dale, Nov 24 2017 *) PROG (PARI) Vec(-(18*x^2-23*x+9)/(x-1)^3 + O(x^100)) \\ Colin Barker, Jan 21 2014 CROSSREFS Cf. A152948, A000217, A000290, A000326, A000384, A000566, A000567, A001106, A001107, A051682, A051624, A051865-A051876. Sequence in context: A010540 A187466 A082695 * A019909 A324002 A342947 Adjacent sequences:  A236254 A236255 A236256 * A236258 A236259 A236260 KEYWORD nonn,easy AUTHOR Vladimir Shevelev, Jan 21 2014 STATUS approved

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Last modified June 21 06:24 EDT 2021. Contains 345358 sequences. (Running on oeis4.)