%I M4181 N1741
%S 1,6,28,120,495,2002,8008,31824,125970,497420,1961256,7726160,
%T 30421755,119759850,471435600,1855967520,7307872110,28781143380,
%U 113380261800,446775310800,1761039350070,6943526580276,27385657281648,108043253365600
%N Binomial coefficients C(2n,n2).
%C Number of lattice paths from (0,0) to (n,n) with steps E=(1,0) and N=(0,1) which touch or cross the line xy=2. Example: For n=3 there are 6 paths EEENNN, EENENN, EENNEN, EENNNE, ENEENN and NEEENN.  _Herbert Kociemba_, May 23 2004
%C Number of dissections of a convex (n+3)gon by noncrossing diagonals into several regions, exactly n2 of which are triangular. Example: a(3)=6 because the convex hexagon ABCDEF is dissected by any of the diagonals AC, BD, CE, DF, EA, FB into regions containing exactly 1 triangle.  _Emeric Deutsch_, May 31 2004
%C Number of UUU's (triple rises), where U=(1,1), in all Dyck paths of semilength n+1. Example: a(3)=6 because we have UD(UUU)DDD, (UUU)DDDUD, (UUU)DUDDD, (UUU)DDUDD and (U[UU)U]DDDD, the triple rises being shown between parentheses.  _Emeric Deutsch_, Jun 03 2004
%C Inverse binomial transform of A026389.  _Ross La Haye_, Mar 05 2005
%C Sum of the jumplengths of all full binary trees with n internal nodes. In the preorder traversal of a full binary tree, any transition from a node at a deeper level to a node on a strictly higher level is called a jump; the positive difference of the levels is called the jump distance; the sum of the jump distances in a given full binary tree is called the jumplength.  _Emeric Deutsch_, Jan 18 2007
%C a(n) = number of convex polyominoes (A005436) of perimeter 2n+4 that are directed but not parallelogram polyominoes, because the directed convex polyominoes are counted by the central binomial coefficient binom(2n,n) and the subset of parallelogram polyominoes is counted by the Catalan number C(n+1) = binom(2n+2,n+1)/(n+2) and a(n) = binom(2n,n)  C(n+1).  _David Callan_, Nov 29 2007
%C a(n) = number of DUU's in all Dyck paths of semilength n+1. Example: a(3)=6 because we have UU(DUU)DDD, U(DUU)UDDD, U(DUU)DUDD, UDU(DUU)DD, U(DUU)DDUD, UUD(DUU)DD, the DUU's being shown between parentheses and no other Dyck path of semilength 4 contains a DUU.  _David Callan_, Jul 25 2008
%C C(2n,nm) is the number of Dycktype walks such that their graphs have one marked edge passed 2m times and the other edges are passed 2 times counting "there and back" directions.  _Oleksiy Khorunzhiy_, Jan 09 2015
%C Number of paths in the halfplane x>=0, from (0,0) to (2n,4), and consisting of steps U=(1,1) and D=(1,1). For example, for n=3, we have the 6 paths: UUUUUD, UUUUDU, UUUDUU, UUDUUU, UDUUUU, DUUUUU, DUUUUU.  _José Luis Ramírez Ramírez_, Apr 19 2015
%D M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 828.
%D C. Lanczos, Applied Analysis. PrenticeHall, Englewood Cliffs, NJ, 1956, p. 517.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H T. D. Noe, <a href="/A002694/b002694.txt">Table of n, a(n) for n=2..200</a>
%H M. Abramowitz and I. A. Stegun, eds., <a href="http://www.convertit.com/Go/ConvertIt/Reference/AMS55.ASP">Handbook of Mathematical Functions</a>, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
%H H. Bottomley, <a href="/A002694/a002694.gif">Illustration for A000108, A001147, A002694, A067310 and A067311</a>
%H Milan Janjic, <a href="http://www.pmfbl.org/janjic/">Two Enumerative Functions</a>
%H M. Janjic and B. Petkovic, <a href="http://arxiv.org/abs/1301.4550">A Counting Function</a>, arXiv preprint arXiv:1301.4550 [math.CO], 2013.  _N. J. A. Sloane_, Feb 13 2013
%H M. Janjic, B. Petkovic, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL17/Janjic/janjic45.html">A Counting Function Generalizing Binomial Coefficients and Some Other Classes of Integers</a>, J. Int. Seq. 17 (2014) # 14.3.5
%H O. Khorunzhiy, <a href="http://arxiv.org/abs/1107.5724">On high moments and the spectral norm of large dilute Wigner random matrices</a>, arXiv:1107.5724 [mathph], 2014.
%H O. Khorunzhiy, <a href="https://doi.org/10.15407/mag10.01.064">On high moments and the spectral norm of large dilute Wigner random matrices</a>, Zh. Mat. Fiz. Anal. Geom. 10 (1) (2014), pp. 64125.
%H W. Krandick, <a href="http://dx.doi.org/10.1016/j.cam.2003.08.018">Trees and jumps and real roots</a>, J. Computational and Applied Math., 162, 2004, 5155.
%H C. Lanczos, <a href="/A002457/a002457.pdf">Applied Analysis</a> (Annotated scans of selected pages)
%H Asamoah Nkwanta and Earl R. Barnes, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL15/Nkwanta/nkwanta2.html">Two Catalantype Riordan Arrays and their Connections to the Chebyshev Polynomials of the First Kind</a>, Journal of Integer Sequences, Article 12.3.3, 2012.  From _N. J. A. Sloane_, Sep 16 2012
%H V. Pilaud, J. Rué, <a href="http://arxiv.org/abs/1307.6440">Analytic combinatorics of chord and hyperchord diagrams with k crossings</a>, arXiv preprint arXiv:1307.6440 [math.CO], 2013; Adv. Appl. Math. 57 (2014) 60100.
%H T. Tao, Van Vu, <a href="https://arxiv.org/abs/1005.2901">Random matrices: localization of the eigenvalues and the necessity of four moments</a>, arXiv:1005.2901 [math.PR]20102011; Acta Math. Vietnam 36 (2) (2011) 431449.
%F a(n) = A067310(n, 1) as this is number of ways of arranging n chords on a circle (handshakes between 2n people across a table) with exactly 1 simple intersection.  _Henry Bottomley_, Oct 07 2002
%F E.g.f.: exp(2*x)*BesselI(2, 2*x).  _Vladeta Jovovic_, Aug 21 2003
%F G.f.: [1sqrt(14z)]^4/[16z^2*sqrt(14z)].  _Emeric Deutsch_, Jan 28 2004
%F a(n) = sum{k=0..n, C(n, k)C(n, k+2)}.  _Paul Barry_, Sep 20 2004
%F (n2)*(n+2)*a(n) +2*n*(2*n1)*a(n1)=0.  _R. J. Mathar_, Dec 04 2012
%F G.f.: z^2*C(z)^4/(12*z*C(z)), where C(z) is the g.f. of Catalan numbers.  _José Luis Ramírez Ramírez_, Apr 19 2015
%F a(n) = Sum_{k=1..n}(binomial(2*nk,nk1)).  _Vladimir Kruchinin_, Oct 22 2016
%p a:=n>sum(binomial(n,j1)*binomial(n,j+1),j=1..n): seq(a(n), n=2..25); # _Zerinvary Lajos_, Nov 26 2006
%t CoefficientList[ Series[ 16/(((Sqrt[1  4 x] + 1)^4)*Sqrt[1  4 x]), {x, 0, 23}], x] (* _Robert G. Wilson v_, Aug 08 2011 *)
%t Table[Binomial[2n,n2],{n,2,30}] (* _Harvey P. Dale_, Jun 12 2014 *)
%o (Haskell)
%o a002694 n = a007318' (2 * n) (n  2)  _Reinhard Zumkeller_, Jun 18 2012
%o (MAGMA) [Binomial(2*n, n2): n in [2..30]]; // _Vincenzo Librandi_, Apr 20 2015
%Y Cf. A006659.
%Y Diagonal 5 of triangle A100257.
%Y Cf. A009766.
%K nonn
%O 2,2
%A _N. J. A. Sloane_.
