Summary.In this article, we formalize in the Mizar system [1,7]
PreliminariesLet X be a non empty set and f be a non-negative partial function from X to R. Observe that −f is non-positive.Let f be a non-positive partial function from X to R. One can check that −f is non-negative. Now we state the propositions: (1) Let us consider a non empty set X, a non-positive partial function f from X to R, and a set E. Then f E is non-positive. (2) Let us consider a non empty set X, a set A, a real number r, and a partial function f from X to R. (ii) if c 0, then c · f is non-negative.