OFFSET
1,1
LINKS
Don Coppersmith, Ponder This: IBM Research Monthly Puzzles, March challenge
Index entries for linear recurrences with constant coefficients, signature (6,-15,20,-15,6,-1).
FORMULA
a(n) = 1/6*n*(n^4+10*n^3+35*n^2+50*n-36). More generally, number of m X n (0, 1) matrices such that each row and each column is increasing or decreasing is 2*n*(2*binomial(n+m-1, n)-m) = 4/Beta(m, n)-2*m*n.
G.f.: -10*x*(x^4-4*x^3+6*x^2-4*x-1) / (x-1)^6. [Colin Barker, Feb 22 2013]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vladimir Baltic, Vladeta Jovovic, Jul 10 2003
STATUS
approved