OFFSET
0,2
MATHEMATICA
a[ 1, 1 ]=1; a[ n_, k_ ] := 0 /; (k>n || k<=0); a[ n_, 1 ]=Sum[ a[ n-1, i ], {i, 1, n-1} ] a[ n_, k_ ] := a[ n, k ]=2a[ n, k-1 ]-a[ n, k-2 ]-a[ n-1, k-1 ]-a[ n-1, k-2 ]; Union[ Flatten[ Table[ a[ n, i ], {n, 1, 20}, {i, 1, n} ] ] ]
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
STATUS
approved