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 A218324 Odd heptagonal pyramidal numbers. 1
 1, 115, 645, 1911, 4233, 7931, 13325, 20735, 30481, 42883, 58261, 76935, 99225, 125451, 155933, 190991, 230945, 276115, 326821, 383383, 446121, 515355, 591405, 674591, 765233, 863651, 970165, 1085095, 1208761, 1341483, 1483581, 1635375, 1797185, 1969331 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Table of n, a(n) for n=1..34. Index entries for linear recurrences with constant coefficients, signature (4, -6, 4, -1). FORMULA a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4). a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) + 320. a(n) = (2*n-1)*(4*n-3)*(20*n-17)/3. G.f.: x*(1 + 111*x + 191*x^2 + 17*x^3)/(1-x)^4. EXAMPLE The sequence of heptagonal pyramidal numbers A002413 begins 1, 8, 26, 60, 115, 196, 308, 456, 645, 880, ... . As the third odd term is 645, a(3) = 645. MATHEMATICA LinearRecurrence[{4, -6, 4, -1}, {1, 115, 645, 1911}, 34] CROSSREFS Cf. A002413, A218325. Sequence in context: A255143 A154070 A251215 * A258673 A287430 A256889 Adjacent sequences: A218321 A218322 A218323 * A218325 A218326 A218327 KEYWORD nonn,easy AUTHOR Ant King, Oct 26 2012 STATUS approved

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Last modified March 2 01:29 EST 2024. Contains 370447 sequences. (Running on oeis4.)