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 A098430 a(n) = 4^n*(2*n)!/(n!)^2. 6
 1, 8, 96, 1280, 17920, 258048, 3784704, 56229888, 843448320, 12745441280, 193730707456, 2958796259328, 45368209309696, 697972450918400, 10768717814169600, 166556168859156480, 2581620617316925440, 40091049586568724480, 623638549124402380800, 9715632133727531827200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) counts walks of 2n steps North, East, South or West that start at the origin and end on the line y=x. For example, a(1)=8 counts EW, EN, NE, NS, WE, WS, SN, SW. If the walk has i East and j North steps, then it must have n-j West and n-i South steps. There are Multinomial[i,j,n-j,n-i] ways to arrange these steps and summing over i and j gives the result. - David Callan, Oct 11 2005 Number of lattice paths from (0,0) to (n,n) using steps (1,0), (0,1), both of two kinds. - Joerg Arndt, Jul 01 2011 Hankel transform is A121913. - Philippe Deléham, Mar 01 2009 Convolving a(n) with itself yields A001025, the powers of 16. Thus the limiting ratio of this sequence is 16. - Bob Selcoe, Jul 16 2014 Number of strings x of length 4n over the alphabet {1, -1} such that the dot product of x with (x reversed) is 0. - Jeffrey Shallit, Mar 06 2017 Number of orthogonal pairs of vectors of length 2n, constructed with any symmetric binary-valued symbol set. - Ross Drewe, May 18 2018 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..200 Hacène Belbachir, Abdelghani Mehdaoui, László Szalay, Diagonal Sums in the Pascal Pyramid, II: Applications, J. Int. Seq., Vol. 22 (2019), Article 19.3.5. Tony D. Noe, On the Divisibility of Generalized Central Trinomial Coefficients, Journal of Integer Sequences, Vol. 9 (2006), Article 06.2.7. FORMULA a(n) = 4^n*binomial(2*n, n) = 4^n*A000984(n). E.g.f.: exp(8*x)*BesselI(0, 8*x). G.f.: 1/sqrt(1-16*x). - Zerinvary Lajos, Dec 20 2008, corrected R. J. Mathar, May 18 2009 a(n) = (1/Pi)*integral(x=-2..2, (2*x)^(2*n)/(sqrt((2-x)*(2+x))). - Peter Luschny, Sep 12 2011 D-finite with recurrence: n*a(n) + 8*(-2*n+1)*a(n-1) = 0. - R. J. Mathar, Nov 10 2014 a(n) = A249308(2*n). - Reinhard Zumkeller, Nov 14 2014 a(n) = 16^n*hypergeometric([-2*n, 1/2], , 2). - Peter Luschny, May 19 2015 a(n) = A174301(2n,n). - Alois P. Heinz, Apr 15 2019 From Amiram Eldar, Jul 21 2020: (Start) Sum_{n>=0} 1/a(n) = 16/15 + 16*sqrt(15)*arcsin(1/4)/225. Sum_{n>=0} (-1)^n/a(n) = 16/17 - 16*sqrt(17)*arcsinh(1/4)/289. (End) MAPLE A098430 := n -> 4^n*binomial(2*n, n): seq(A098430(n), n=0..30); # Wesley Ivan Hurt, Jul 16 2014 MATHEMATICA CoefficientList[Series[1/Sqrt[1 - 16 x], {x, 0, 16}], x] (* Robert G. Wilson v, Jun 28 2012 *) Table[4^n(2n)!/(n!)^2, {n, 0, 20}] (* Harvey P. Dale, Aug 13 2021 *) PROG (PARI) /* as lattice paths: same as in A092566 but use */ steps=[[1, 0], [1, 0], [0, 1], [0, 1]]; /* note the double [1, 0] and [0, 1] */ /* Joerg Arndt, Jul 01 2011 */ (MAGMA) [4^n*Factorial(2*n)/Factorial(n)^2: n in [0..20]]; // Vincenzo Librandi, Jul 05 2011 (Haskell) a098430 n = a000302 n * a000984 n -- Reinhard Zumkeller, Nov 14 2014 (Sage) a = lambda n: 16^n*hypergeometric([-2*n, 1/2], , 2) [simplify(a(n)) for n in range(23)] # Peter Luschny, May 19 2015 CROSSREFS Cf. A000302, A000984, A174301, A249308. Sequence in context: A060458 A173834 A260627 * A220285 A052570 A034177 Adjacent sequences:  A098427 A098428 A098429 * A098431 A098432 A098433 KEYWORD easy,nonn AUTHOR Paul Barry, Sep 07 2004 STATUS approved

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Last modified January 22 01:28 EST 2022. Contains 350481 sequences. (Running on oeis4.)