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 A033570 Pentagonal numbers with odd index: a(n) = (2*n+1)*(3*n+1). 20
 1, 12, 35, 70, 117, 176, 247, 330, 425, 532, 651, 782, 925, 1080, 1247, 1426, 1617, 1820, 2035, 2262, 2501, 2752, 3015, 3290, 3577, 3876, 4187, 4510, 4845, 5192, 5551, 5922, 6305, 6700, 7107, 7526, 7957, 8400, 8855, 9322, 9801, 10292, 10795, 11310, 11837 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS If Y is a 3-subset of an 2*n-set X then, for n >= 4, a(n-2) is the number of 4-subsets of X having at least two elements in common with Y. - Milan Janjic, Dec 16 2007 Sequence found by reading the line (one of the diagonal axes) from 1, in the direction 1, 12, ..., in the square spiral whose vertices are the generalized pentagonal numbers A001318. - Omar E. Pol, Sep 08 2011 If two independent real random variables, x and y, are distributed according to the same exponential distribution: pdf(x) = lambda * exp(-lambda * x), lambda > 0, then the probability that 2 <= x/(n*y) < 3 is given by n/a(n) (for n>1). - Andres Cicuttin, Dec 11 2016 a(n) is the sum of 2*n+1 consecutive integers starting from 2*n+1. - Bruno Berselli, Jan 16 2018 LINKS Michael De Vlieger, Table of n, a(n) for n = 0..10000 John Elias, Illustration: Natural number stars. Leo Tavares, Illustration: Square Block Triangles Eric Weisstein's World of Mathematics, Pentagonal Number. Wikipedia, Pentagonal number. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA G.f.: (1 + 9*x + 2*x^2)/(1-x)^3. a(n) = a(n-1) + 12*n-1 for n > 0, a(0)=1. - Vincenzo Librandi, Nov 17 2010 a(n) = A000326(2*n+1) = A191967(2*n+1). - Reinhard Zumkeller, Jul 07 2012 a(n) = Sum_{i=1..2*(n+1)-1} 4*(n+1) - 2 - i. - Wesley Ivan Hurt, Mar 18 2014 E.g.f.: (1 + 11*x + 6*x^2)*exp(x). - G. C. Greubel, Oct 12 2019 From Amiram Eldar, Feb 20 2022: (Start) Sum_{n>=0} 1/a(n) = Pi/(2*sqrt(3)) - 2*log(2) + 3*log(3)/2. Sum_{n>=0} (-1)^n/a(n) = (1/sqrt(3) - 1/2)*Pi + log(2). (End) a(n) = A016754(n) + A014105(n). - Leo Tavares, May 24 2022 MAPLE A033570:=n->(2*n+1)*(3*n+1); seq(A033570(n), n=0..40); # Wesley Ivan Hurt, Mar 18 2014 MATHEMATICA LinearRecurrence[{3, -3, 1}, {1, 12, 35}, 50] Table[(2 n + 1) (3 n + 1), {n, 0, 50}] (* or *) CoefficientList[Series[(1 + 9 x + 2 x^2)/(1 - x)^3, {x, 0, 50}], x] (* Michael De Vlieger, Dec 12 2016 *) PolygonalNumber[5, Range[1, 101, 2]] (* Harvey P. Dale, Aug 02 2021 *) PROG (PARI) a(n)=(2*n+1)*(3*n+1) \\ Charles R Greathouse IV, Jun 11 2015 (Magma) [(2*n+1)*(3*n+1) : n in [0..50]]; // Wesley Ivan Hurt, Dec 11 2016 (Sage) [(2*n+1)*(3*n+1) for n in range(50)] # G. C. Greubel, Oct 12 2019 (GAP) List([0..50], n-> (2*n+1)*(3*n+1)); # G. C. Greubel, Oct 12 2019 CROSSREFS Cf. A000326, A001318, A033568, A049452, A049453, A191967. Cf. A016754, A014105. Sequence in context: A077293 A053682 A280364 * A163661 A247893 A348462 Adjacent sequences: A033567 A033568 A033569 * A033571 A033572 A033573 KEYWORD nonn,easy AUTHOR N. J. A. Sloane EXTENSIONS More terms from Ray Chandler, Dec 08 2011 STATUS approved

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Last modified June 9 08:19 EDT 2023. Contains 363168 sequences. (Running on oeis4.)