Consider the linear programming problem:
   Maximize  cx
   subject to Ax=b, x ? 0, where A is an m x n matrix of rank m.
Suppose that an optimal solution with basis B is at hand. Further
suppose that b is replaced by b + ?d where ? is a scalar and d is a
fixed nonzero vector of dimension m. Give a condition such that the
basis B will be optimal for all ? ? 0