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 A218328 Odd 9-gonal (nonagonal) pyramidal numbers. 1
 1, 155, 885, 2639, 5865, 11011, 18525, 28855, 42449, 59755, 81221, 107295, 138425, 175059, 217645, 266631, 322465, 385595, 456469, 535535, 623241, 720035, 826365, 942679, 1069425, 1207051, 1356005, 1516735, 1689689, 1875315, 2074061, 2286375, 2512705 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Table of n, a(n) for n=1..33. 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) + 448. a(n) = (2*n-1)*(4*n-3)*(28*n-25)/3. G.f.: x*(1+151*x+271*x^2+25*x^3)/(1-x)^4. EXAMPLE The sequence of 9-gonal (nonagonal) pyramidal numbers A007584 begins 1, 10, 34, 80, 155, 266, 420, 624, 885, 1210, .... As the third odd term is 885, then a(3) = 885. MATHEMATICA LinearRecurrence[{4, -6, 4, -1}, {1, 155, 885, 2639}, 33] PROG (PARI) a(n)=(2*n-1)*(4*n-3)*(28*n-25)/3 \\ Charles R Greathouse IV, Oct 18 2022 CROSSREFS Cf. A007584, A218329. Sequence in context: A028382 A250936 A224336 * A101535 A010086 A110834 Adjacent sequences: A218325 A218326 A218327 * A218329 A218330 A218331 KEYWORD nonn,easy AUTHOR Ant King, Oct 28 2012 STATUS approved

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Last modified April 12 16:45 EDT 2024. Contains 371635 sequences. (Running on oeis4.)