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 A131235 Triangle read by rows: T(n,k) is number of (n-k) X k matrices, k=0..n, with nonnegative integer entries and every row and column sum <= 2. 5
 1, 1, 1, 1, 3, 1, 1, 6, 6, 1, 1, 10, 26, 10, 1, 1, 15, 79, 79, 15, 1, 1, 21, 189, 451, 189, 21, 1, 1, 28, 386, 1837, 1837, 386, 28, 1, 1, 36, 706, 5776, 12951, 5776, 706, 36, 1, 1, 45, 1191, 15085, 66021, 66021, 15085, 1191, 45, 1, 1, 55, 1889, 34399, 258355, 551681 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS Row sums give A131236. REFERENCES R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.65(a). LINKS FORMULA G.f. column 2: (-1-x-6*x^2+x^3+x^4)/(x-1)^5. -  R. J. Mathar, Mar 20 2018 T(n,2) = (4+8*n+5*n^2+6*n^3+n^4)/4. -  R. J. Mathar, Mar 20 2018 G.f. column 3: -(1+3*x+30*x^2+73*x^3+24*x^4-48*x^5+7*x^6)/(x-1)^7 . -  R. J. Mathar, Mar 20 2018 T(n,3) = (8+58*n^2+3*n^3+n^4+9*n^5+n^6)/8. - R. J. Mathar, Mar 20 2018 EXAMPLE 1; 1,1; 1,3,1; 1,6,6,1; 1,10,26,10,1; 1,15,79,79,15,1; 1,21,189,451,189,21,1; ... or as a symmetric array 1   1    1   1   1  1 1 ... 1   3    6  10  15 21 ... 1   6   26  79 189 .. 1  10   79 451 .. 1  15  189 .. 1  21 .. MAPLE A131235 := proc(m, n)    exp((x*y*(3-x*y)+(x+y)*(2-x*y))/2/(1-x*y))/sqrt(1-x*y) ;    coeftayl(%, y=0, n)*n!;    coeftayl(%, x=0, m)*m! ; end proc: # R. J. Mathar, Mar 20 2018 CROSSREFS Cf. A049088 (diagonal), A131236, A131237, A088699 and A086885 (sums <= 1), A000217 (column 1) Sequence in context: A100862 A098568 A180959 * A202812 A157243 A146769 Adjacent sequences:  A131232 A131233 A131234 * A131236 A131237 A131238 KEYWORD nonn,tabl AUTHOR Vladeta Jovovic, Jun 20 2007 STATUS approved

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Last modified June 25 06:20 EDT 2021. Contains 345452 sequences. (Running on oeis4.)