%I A001501 M5175 N2247
%S A001501 1,0,0,1,24,2040,297200,68938800,24046189440,12025780892160,
%T A001501 8302816499443200,7673688777463632000,9254768770160124288000,
%U A001501 14255616537578735986867200,27537152449960680597739468800
%N A001501 Number of n X n 0-1 matrices with all column and row sums equal to 3.
%D A001501 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A001501 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A001501 L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 236, P(n,3).
%D A001501 R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, 
               Department of Mathematics, Univ. London, 1958.
%D A001501 R. P. Stanley, Enumerative Combinatorics, Wadsworth, Vol. 1, 1986; see 
               Example 1.1.3, page 2, f(n).
%D A001501 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.
%H A001501 T. D. Noe, <a href="b001501.txt">Table of n, a(n) for n=0..50</a>
%H A001501 <a href="Sindx_Mat.html#binmat">Index entries for sequences related to 
               binary matrices</a>
%o A001501 (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)
%Y A001501 Sequence in context: A006685 A002671 A003738 this_sequence A054005 A107675 
               A008977
%Y A001501 Adjacent sequences: A001498 A001499 A001500 this_sequence A001502 A001503 
               A001504
%K A001501 nonn,nice
%O A001501 0,5
%A A001501 N. J. A. Sloane (njas(AT)research.att.com).
%E A001501 Additional comments from Michael Somos, May 28, 2002