Proceedings of the American Mathematical Society, Vol. 27, No. 1 (Jan., 1971), pp. 13-15 (3 pages) How should one select an l-element subset of a rectangular array of lattice points (points with ...

When the order of the operations influences the end result, the matrices are called “non-commutative.” In this case, the product Q1 x Q2 differs from the product Q2 x Q1.