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 A028329 Twice central binomial coefficients. 15
 2, 4, 12, 40, 140, 504, 1848, 6864, 25740, 97240, 369512, 1410864, 5408312, 20801200, 80233200, 310235040, 1202160780, 4667212440, 18150270600, 70690527600, 275693057640, 1076515748880, 4208197927440, 16466861455200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Central elements in the even-Pascal triangle A028326. If Y is a 3-subset of an 2n-set X then, for n>=3, a(n-1) is the number of (n+1)-subsets of X having at least two elements in common with Y. - Milan Janjic, Dec 16 2007 a(n) denotes the number of ways one can reach the (n,n) point in an n X n grid via the point (n-1, n-1) starting from (0,0) when moving right and up is allowed [From Avik Roy (avik_3.1416(AT)yahoo.co.in), Jan 29 2009] It appears that a(n-1) is also the number of quivers in the mutation class of twisted types BD_n and CD_n for n >= 3. - Christian Stump, Nov 03 2010 This is the case m = n+1 in the Catalan's formula (2m)!*(2n)!/(m!*(m+n)!*n!) - see Umberto Scarpis in References. - Bruno Berselli, Apr 27 2012 a(n) is the number of North-East paths from (0,0) to (n+1,n+1) that bounce off the diagonal y = x an even number of times. Details can be found in Section 4.2 in Pan and Remmel's link. - Ran Pan, Feb 01 2016 a(n) is the number of North-East paths from (0,0) to (n+1,n+1) that cross the diagonal y = x an even number of times. Details can be found in Section 4.3 in Pan and Remmel's link. - Ran Pan, Feb 01 2016 REFERENCES Umberto Scarpis, Sui numeri primi e sui problemi dell'analisi indeterminata in Questioni riguardanti le matematiche elementari, Nicola Zanichelli Editore (1924-1927, third Edition), page 11. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..300 Guo-Niu Han, Enumeration of Standard Puzzles, 2011. [Cached copy] Guo-Niu Han, Enumeration of Standard Puzzles, arXiv:2006.14070 [math.CO], 2020. Ran Pan and Jeffrey B. Remmel, Paired patterns in lattice paths, arXiv:1601.07988 [math.CO], 2016. FORMULA G.f.: 2/sqrt(1 - 4*x). a(n) = 2*A000984(n). a(n) = 2 * binomial(2*n, n). a(n) = A100320(n) = A095660(2*n,n) for n > 0. - Reinhard Zumkeller, Apr 08 2012 G.f.: G(0), where G(k)= 1 + 1/(1 - 2*x*(2*k + 1)/(2*x*(2*k + 1) + (k + 1)/ G(k+1))); (continued fraction). - Sergei N. Gladkovskii, Jun 07 2013 a(n) = binomial(2*n+2, n+1) - A162551(n). - Ran Pan, Feb 01 2016 D-finite with recurrence: n*a(n) + 2*(-2*n+1)*a(n-1)=0. - R. J. Mathar, Jan 17 2020 MAPLE seq(add(binomial(2*n, n), k=1..2), n=0..23); # Zerinvary Lajos, Dec 14 2007 MATHEMATICA Table[2Binomial[2n, n], {n, 0, 30}] (* Harvey P. Dale, Aug 08 2011 *) PROG (PARI) a(n)=2*binomial(2*n, n) CROSSREFS Bisection of A047073, A063886. First differences of A054113. Cf. A000984, A095660, A100320, A162551. Sequence in context: A327845 A056236 A300652 * A204678 A025227 A211965 Adjacent sequences:  A028326 A028327 A028328 * A028330 A028331 A028332 KEYWORD nonn,easy AUTHOR EXTENSIONS Edited by Michael Somos, Sep 13 2003 STATUS approved

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Last modified October 26 20:44 EDT 2020. Contains 338027 sequences. (Running on oeis4.)