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 A195317 Centered 40-gonal numbers. 10
 1, 41, 121, 241, 401, 601, 841, 1121, 1441, 1801, 2201, 2641, 3121, 3641, 4201, 4801, 5441, 6121, 6841, 7601, 8401, 9241, 10121, 11041, 12001, 13001, 14041, 15121, 16241, 17401, 18601, 19841, 21121, 22441, 23801, 25201, 26641, 28121, 29641, 31201, 32801 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also centered tetracontagonal numbers or centered tetrakaicontagonal numbers. Also sequence found by reading the line from 1, in the direction 1, 41, ..., in the square spiral whose vertices are the generalized dodecagonal numbers A195162. Semi-axis opposite to A195322 in the same spiral. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..10000 Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = 20*n^2 - 20*n + 1. Sum_{n>=1} 1/a(n) = Pi*tan(Pi/sqrt(5))/(8*sqrt(5)). - Amiram Eldar, Feb 11 2022 MATHEMATICA Table[20*n^2 - 20*n + 1, {n, 1, 40}] (* Amiram Eldar, Feb 11 2022 *) PROG (Magma) [20*n^2 - 20*n + 1: n in [1..50]]; // Vincenzo Librandi, Sep 21 2011 (PARI) a(n)=20*n^2-20*n+1 \\ Charles R Greathouse IV, Jun 17 2017 CROSSREFS Bisection of A195148. Cf. A003154, A069129, A069133, A069190, A195314, A195315, A195316, A195318. Sequence in context: A266954 A290589 A191867 * A044292 A044673 A135793 Adjacent sequences:  A195314 A195315 A195316 * A195318 A195319 A195320 KEYWORD nonn,easy AUTHOR Omar E. Pol, Sep 16 2011 STATUS approved

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Last modified May 25 09:37 EDT 2022. Contains 354066 sequences. (Running on oeis4.)