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A218324
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Odd heptagonal pyramidal numbers.
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1
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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
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OFFSET
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1,2
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LINKS
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FORMULA
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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.
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EXAMPLE
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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.
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MATHEMATICA
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LinearRecurrence[{4, -6, 4, -1}, {1, 115, 645, 1911}, 34]
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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