0,5

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 236, P(n,3).

R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, Department of Mathematics, Univ. London, 1958.

R. P. Stanley, Enumerative Combinatorics, Wadsworth, Vol. 1, 1986; see Example 1.1.3, page 2, f(n).

M. L. Stein and P. R. Stein, Enumeration of Stochastic Matrices with Integer Elements. Report LA-4434, Los Alamos Scientific Laboratory of the University of California, Los Alamos, NM, Jun 1970.

T. D. Noe, Table of n, a(n) for n=0..50

Index entries for sequences related to binary matrices

(PARI) a(n)=local(k); if(n<0, 0, n!^2*sum(j=0, n, sum(i=0, n-j, if(1, k=n-i-j; (j+3*k)!/(3^i*36^k*i!*k!^2)))/j!/(-2)^j)) (from Michael Somos)

Sequence in context: A006685 A002671 A003738 this_sequence A054005 A107675 A008977

Adjacent sequences: A001498 A001499 A001500 this_sequence A001502 A001503 A001504

nonn,nice

N. J. A. Sloane (njas(AT)research.att.com).

Additional comments from Michael Somos, May 28, 2002