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 A051288 Triangle read by rows: T(n,k) = number of paths of n upsteps U and n downsteps D that contain k UUDs. 11
 1, 2, 4, 2, 8, 12, 16, 48, 6, 32, 160, 60, 64, 480, 360, 20, 128, 1344, 1680, 280, 256, 3584, 6720, 2240, 70, 512, 9216, 24192, 13440, 1260, 1024, 23040, 80640, 67200, 12600, 252, 2048, 56320, 253440, 295680, 92400, 5544, 4096, 135168, 760320 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS By reading paths backward, the UUD in the name could be replaced by DDU. Or, triangular array T read by rows: T(n,k)=P(2n,n,4k), where P(n,k,c)=number of vectors (x(1),x(2,),...,x(n)) of k 1's and n-k 0's such that x(i)=x(n+1-i) for exactly c values of i. P(n,k,n) counts palindromes. In nuclear magnetic resonance of n coupled spin-1/2 nuclides, T(n,k) is the number of zero-quantum transitions with combination index k. See the [Sykora (2007)] link, containing also yet another interpretation in terms of pairs of binary n-tuples. - Stanislav Sykora, Apr 27 2012 Let u - (u_1, u_2, u_3, ..., u_{2n}) be a binary vector containing n 0's and n 1's. Define a mismatch to be an adjacent pair (u_{2i-1}, u_{2i}) which is neither 0,1 nor 1,0 (think "socks"). Then T(n,k) = number of u's with k mismatches. - N. J. A. Sloane, Nov 03 2017 following an email from Bill Gosper From Colin Defant, Sep 16 2018: (Start) Let s denote West's stack-sorting map. T(n,k) is the number of permutations pi of [n] with k valleys such that s(pi) avoids the patterns 132, 231, and 321. T(n,k) is also the number of permutations pi of [n] with k valleys such that s(pi) avoids the patterns 132, 312, and 321. T(n,k) is the number of permutations of [n] with k valleys that avoid the patterns 1342, 3142, 3412, and 3421. (End) LINKS Stanislav Sykora, Table of n, a(n) for n = 0..2600 Colin Defant, Stack-sorting preimages of permutation classes, arXiv:1809.03123 [math.CO], 2018. Rui Duarte and António Guedes de Oliveira, A Famous Identity of Hajós in Terms of Sets, Journal of Integer Sequences, Vol. 17 (2014), Article 14.9.1. Aristidis Sapounakis, Panagiotis Tsikouras, Ioannis Tasoulas, Kostas Manes, Strings of Length 3 in Grand-Dyck Paths and the Chung-Feller Property, Electr. J. Combinatorics, 19 (2012), #P2. Stanislav Sykora, Triangle T(n,k) for rows n = 0..100 Stanislav Sykora, p-Quantum Transitions and a Combinatorial Identity, Stan's Library II, 2007, Identity (1) for p=0. Stanislav Sýkora, Magnetic Resonance on OEIS, Stan's NMR Blog (Dec 31, 2014), Retrieved Nov 12, 2019. FORMULA T(n, k) = binomial(n, 2*k)*2^(n-2*k)*binomial(2*k, k). G.f.: (1-4*x+4*x^2*(1-y))^(-1/2) = Sum_{n>=0, k>=0} a(n, k)*x^n*y^k. EXAMPLE Table begins n | k=0    1    2    3 --+------------------- 0 |   1 1 |   2 2 |   4    2 3 |   8   12 4 |  16   48    6 5 |  32  160   60 6 |  64  480  360   20 7 | 128 1344 1680  280 ... a(2,1)=2 because UUDD, DUUD each have one UUD. MATHEMATICA Table[Binomial[n, 2k]2^(n-2k)Binomial[2k, k], {n, 0, 15}, {k, 0, n/2}] CROSSREFS Row sums are the (even) central binomial coefficients A000984. A091894 gives the distribution of the parameter "number of DDUs" on Dyck paths. Sequence in context: A114593 A114655 A228890 * A120434 A319030 A285335 Adjacent sequences:  A051285 A051286 A051287 * A051289 A051290 A051291 KEYWORD nonn,tabf AUTHOR EXTENSIONS Additional comments from David Callan, Aug 28 2004 STATUS approved

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Last modified June 7 05:20 EDT 2020. Contains 334837 sequences. (Running on oeis4.)