a(3)=12,因为有12个可能的3X3矩阵,其中包含非负整数项、行和1,2,3和列和1,2,3:
[ 0 0 1 ] [ 0 0 1 ] [ 0 0 1 ] [ 0 0 1 ]
[ 0 0 2 ] [ 0 1 1 ] [ 0 2 0 ] [ 1 0 1 ]
[ 1 2 0 ], [ 1 1 1 ], [ 1 0 2 ], [ 0 2 1 ],
.
[ 0 0 1 ] [ 0 1 0 ] [ 0 1 0 ] [ 0 1 0 ]
[ 1 1 0 ] [ 0 0 2 ] [ 0 1 1 ] [ 1 0 1 ]
[ 0 1 2 ], [ 1 1 1 ], [ 1 0 2 ], [ 0 1 2 ],
.
[ 0 1 0 ] [ 1 0 0 ] [ 1 0 0 ] [ 1 0 0 ]
[ 1 1 0 ] [ 0 0 2 ] [ 0 1 1 ] [ 0 2 0 ]
[ 0 0 3 ], [ 0 2 1 ], [ 0 1 2 ], [ 0 0 3 ].