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 A195147 Concentric 18-gonal numbers. 13
 0, 1, 18, 37, 72, 109, 162, 217, 288, 361, 450, 541, 648, 757, 882, 1009, 1152, 1297, 1458, 1621, 1800, 1981, 2178, 2377, 2592, 2809, 3042, 3277, 3528, 3781, 4050, 4321, 4608, 4897, 5202, 5509, 5832, 6157, 6498, 6841, 7200, 7561, 7938, 8317, 8712, 9109 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Also concentric octadecagonal numbers or concentric octakaidecagonal numbers. Also sequence found by reading the line from 0, in the direction 0, 18, ..., and the same line from 1, in the direction 1, 37, ..., in the square spiral whose vertices are the generalized hendecagonal numbers A195160. Main axis, perpendicular to A027468 in the same spiral. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..10000 Index entries for linear recurrences with constant coefficients, signature (2,0,-2,1). FORMULA G.f.: -x*(1+16*x+x^2) / ( (1+x)*(x-1)^3 ). - R. J. Mathar, Sep 18 2011 From Vincenzo Librandi, Sep 27 2011: (Start) a(n) = (18*n^2 + 7*(-1)^n - 7)/4; a(n) = -a(n-1) + 9*n^2 - 9*n + 1. (End) PROG (MAGMA) [(18*n^2+7*(-1)^n-7)/4: n in [0..50]]; // Vincenzo Librandi, Sep 27 2011 (PARI) a(n)=(18*n^2+7*(-1)^n-7)/4 \\ Charles R Greathouse IV, Sep 28 2015 CROSSREFS Cf. A195321 and A195316 interleaved. Cf. A032528, A077221, A195142, A195143, A195145, A195146, A195148, A195149. Cf. A032527, A195047, A195048. Column 18 of A195040. - Omar E. Pol, Sep 29 2011 Sequence in context: A041638 A041636 A212428 * A198276 A041640 A041642 Adjacent sequences:  A195144 A195145 A195146 * A195148 A195149 A195150 KEYWORD nonn,easy AUTHOR Omar E. Pol, Sep 17 2011 STATUS approved

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Last modified October 18 14:52 EDT 2019. Contains 328161 sequences. (Running on oeis4.)