A351584 - OEIS

A351584

Triangle read by rows: T(n,k) = A(k,n-k), 1 <= k < n, 2 <= n, where A(m,n) is the number of distinct strings consisting of one X, 2*m Y's and 2*n Z's in which the X lies to the right of at least m Y's and at least n Z's.

16, 53, 53, 124, 306, 124, 240, 1103, 1103, 240, 412, 3043, 5664, 3043, 412, 651, 7056, 21095, 21095, 7056, 651, 968, 14476, 63480, 101950, 63480, 14476, 968, 1374, 27114, 163986, 386249, 386249, 163986, 27114, 1374, 1880, 47331, 377616, 1226540, 1798776, 1226540, 377616, 47331, 1880

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OFFSET

2,1

COMMENTS

The general string enumeration problem of counting strings with k+k'-1 X's, m+m' Y's and n+n' Z's in which the k'th X is placed after at least m of the Y's and n of the Z's may be expressed in terms of an integral of incomplete Beta functions and evaluated in terms of Kampe de Feriet functions (see Connor & Fewster, 2022). Other special cases include A351583 and A351585.

LINKS

Table of n, a(n) for n=2..46.

Stephen B. Connor and Christopher J. Fewster, Integrals of incomplete beta functions, with applications to order statistics, random walks and string enumeration, Brazilian Journal of Probability and Statistics 2022, Vol. 36, No. 1, 185-198; arXiv version, arXiv:2104.12216 [math.CA], 201.

FORMULA

T(n,k) = (4*(n - k)*k + 3*n + 2)*binomial(2*n + 2, 2*k + 1)/(4*n + 4) - n*binomial(n, k)^2/2. See Connor & Fewster (2022).

EXAMPLE

Triangle starts:

16;

53, 53;

124, 306, 124;

240, 1103, 1103, 240;

412, 3043, 5664, 3043, 412;