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 A195148 Concentric 20-gonal numbers. 13
 0, 1, 20, 41, 80, 121, 180, 241, 320, 401, 500, 601, 720, 841, 980, 1121, 1280, 1441, 1620, 1801, 2000, 2201, 2420, 2641, 2880, 3121, 3380, 3641, 3920, 4201, 4500, 4801, 5120, 5441, 5780, 6121, 6480, 6841, 7220, 7601, 8000, 8401, 8820, 9241, 9680, 10121 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Also concentric icosagonal numbers. Also sequence found by reading the line from 0, in the direction 0, 20, ..., and the same line from 1, in the direction 1, 41, ..., in the square spiral whose vertices are the generalized dodecagonal numbers A195162. Main axis, perpendicular to A124080 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 From Vincenzo Librandi, Sep 27 2011: (Start) a(n) = 5*n^2 + 2*(-1)^n-2; a(n) = -a(n-1) + 10*n^2 - 10*n + 1. (End) G.f.: x*(1+18*x+x^2)/((1+x)*(1-x)^3). - Bruno Berselli, Sep 27 2011 MATHEMATICA LinearRecurrence[{2, 0, -2, 1}, {0, 1, 20, 41}, 50] (* Harvey P. Dale, Apr 08 2016 *) PROG (MAGMA) [5*n^2+2*(-1)^n-2: n in [0..50]]; // Vincenzo Librandi, Sep 27 2011 (PARI) a(n)=5*n^2+2*(-1)^n-2 \\ Charles R Greathouse IV, Sep 28 2015 CROSSREFS Cf. A195322 and A195317 interleaved. Cf. A032528, A077221, A195142, A195143, A195145, A195146, A195147, A195149. Cf. A032527, A195048, A195049. Column 20 of A195040. - Omar E. Pol, Sep 29 2011 Sequence in context: A041790 A041788 A041786 * A171389 A038639 A041792 Adjacent sequences:  A195145 A195146 A195147 * A195149 A195150 A195151 KEYWORD nonn,easy AUTHOR Omar E. Pol, Sep 17 2011 STATUS approved

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Last modified October 20 11:18 EDT 2019. Contains 328257 sequences. (Running on oeis4.)