 Linear programming sensitivity within a fixed optimal basis

For a linear program with unchanged cost and constraint matrix, an optimal simplex basis remains optimal while its basic solution remains feasible. Its reduced costs and dual certificate do not change. The new basic variables are $A_B^{-1}(b+ϵ)$, and the objective is the displayed affine function, with $y$ the basis's dual solution. This gives an exact piecewise-affine sensitivity region, determined by nonnegativity of the basic variables, rather than merely a formal first derivative.