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 A051870 18-gonal (or octadecagonal) numbers: a(n) = n*(8*n-7). 24
 0, 1, 18, 51, 100, 165, 246, 343, 456, 585, 730, 891, 1068, 1261, 1470, 1695, 1936, 2193, 2466, 2755, 3060, 3381, 3718, 4071, 4440, 4825, 5226, 5643, 6076, 6525, 6990, 7471, 7968, 8481, 9010, 9555, 10116, 10693, 11286, 11895, 12520 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Also, sequence found by reading the segment (0, 1) together with the line from 1, in the direction 1, 18, ..., in the square spiral whose vertices are the triangular numbers A000217. - Omar E. Pol, Apr 26 2008 This sequence does not contain any triangular numbers other than 0 and 1. See A188892. - T. D. Noe, Apr 13 2011 Also sequence found by reading the line from 0, in the direction 0, 18, ... and the parallel line from 1, in the direction 1, 51, ..., in the square spiral whose vertices are the generalized 18-gonal numbers. - Omar E. Pol, Jul 18 2012 Partial sums of 16n + 1 (with offset 0), compare A005570. - Jeremy Gardiner, Aug 04 2012 All x values for Diophantine equation 32*x + 49 = y^2 are given by this sequence and A139278. - Bruno Berselli, Nov 11 2014 This is also a star enneagonal number: a(n) = A001106(n) + 9*A000217(n-1). - Luciano Ancora, Mar 30 2015 REFERENCES Albert H. Beiler, Recreations in the Theory of Numbers, Dover, N.Y., 1964, p. 189. Elena Deza and Michel Marie Deza, Figurate numbers, World Scientific Publishing, 2012, page 6. LINKS Jeremy Gardiner, Table of n, a(n) for n = 0..999 Omar E. Pol, Determinacion geometrica de los numeros primos y perfectos. Wikipedia, Polygonal number. Index to sequences related to polygonal numbers Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA G.f.: x*(1+15*x)/(1-x)^3. - Bruno Berselli, Feb 04 2011 a(n) = 16*n + a(n-1) - 15, with n > 0, a(0) = 0. - Vincenzo Librandi, Aug 06 2010 a(16*a(n)+121*n+1) = a(16*a(n)+121*n) + a(16*n+1). - Vladimir Shevelev, Jan 24 2014 E.g.f.: (8*x^2 + x)*exp(x). - G. C. Greubel, Jul 18 2017 Sum_{n>=1} 1/a(n) = ((1+sqrt(2))*Pi + 2*sqrt(2)*arccoth(sqrt(2)) + 8*log(2))/14. - Amiram Eldar, Oct 20 2020 Product_{n>=2} (1 - 1/a(n)) = 8/9. - Amiram Eldar, Jan 22 2021 MAPLE A051870 := proc(n) n*(8*n-7) ; end proc: seq(A051870(n), n=0..30) ; # R. J. Mathar, Feb 05 2011 MATHEMATICA Table[n (8 n - 7), {n, 0, 40}] (* Bruno Berselli, Nov 11 2014 *) PROG (PARI) a(n)=n*(8*n-7) \\ Charles R Greathouse IV, Jul 19 2011 CROSSREFS Cf. A014634, A014635, A033585, A033586, A033587, A035008, A069129, A085250, A129271-A129278, A139278. Sequence in context: A273459 A092068 A093520 * A262454 A175815 A069130 Adjacent sequences: A051867 A051868 A051869 * A051871 A051872 A051873 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Dec 15 1999 STATUS approved

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Last modified June 14 20:28 EDT 2024. Contains 373401 sequences. (Running on oeis4.)