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 A045945 Hexagonal matchstick numbers: a(n) = 3*n*(3*n+1). 11
 0, 12, 42, 90, 156, 240, 342, 462, 600, 756, 930, 1122, 1332, 1560, 1806, 2070, 2352, 2652, 2970, 3306, 3660, 4032, 4422, 4830, 5256, 5700, 6162, 6642, 7140, 7656, 8190, 8742, 9312, 9900, 10506, 11130, 11772, 12432, 13110, 13806, 14520, 15252, 16002 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS This may also be construed as the number of line segments illustrating the isometric projection of a cube of side length n. Moreover, a(n) equals the number of rods making a cube of side length n+1 minus the number of rods making a cube of side length n. See the illustration in the links and formula below. LINKS Ivan Panchenko, Table of n, a(n) for n = 0..1000 Amelia Carolina Sparavigna, The groupoids of Mersenne, Fermat, Cullen, Woodall and other Numbers and their representations by means of integer sequences, Politecnico di Torino, Italy (2019), [math.NT]. Index entries for linear recurrences with constant coefficients, signature (3,-3,1). FORMULA a(n) = a(n-1) + 6*(3*n-1) (with a(0)=0). - Vincenzo Librandi, Nov 18 2010 G.f.: 6*x*(2+x)/(1-x)^3. - Colin Barker, Feb 12 2012 a(n) = 6*A005449(n). - R. J. Mathar, Feb 13 2016 a(n) = A059986(n) - A059986(n-1). - Peter M. Chema, Feb 26 2017 a(n) = 6*(A000217(n) + A000290(n)). - Peter M. Chema, Mar 26 2017 From Amiram Eldar, Jan 14 2021: (Start) Sum_{n>=1} 1/a(n) = 1 - Pi/(6*sqrt(3)) - log(3)/2. Sum_{n>=1} (-1)^(n+1)/a(n) = -1 + Pi/(3*sqrt(3)) + 2*log(2)/3. (End) MAPLE a:= n-> 3*n*(3*n+1): seq(a(n), n=0..42); # Zerinvary Lajos, May 03 2007 MATHEMATICA f[n_]:=3*n*(3*n+1); f[Range[0, 60]] (* Vladimir Joseph Stephan Orlovsky, Feb 05 2011 *) PROG (PARI) a(n) = 3*n*(3*n+1) \\ Charles R Greathouse IV, Feb 27 2017 (Python) def a(n): return 3*n*(3*n+1) # Indranil Ghosh, Mar 26 2017 CROSSREFS Cf. A033580, A045946, A059986. The hexagon matchstick sequences are: Number of matchsticks: this sequence; size=1 triangles: A033581; larger triangles: A307253; total triangles: A045949.  Analog for triangles: A045943; analog for stars: A045946. - John King, Apr 05 2019 Sequence in context: A335150 A270700 A282693 * A210206 A005901 A090554 Adjacent sequences:  A045942 A045943 A045944 * A045946 A045947 A045948 KEYWORD nonn,easy,changed AUTHOR STATUS approved

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Last modified January 24 19:12 EST 2021. Contains 340411 sequences. (Running on oeis4.)