|S(a，M 1)f(x)dx-S(a，M2)f(x)dx|=|S(M 1，M2)f(x)dx | & lt； The fifth letter of the Greek alphabet.

Then S(a, oo)f(x)dx converges. Where S(a, oo) represents a generalized integral from a to infinity.

2) Point set A is understood as several sets (the size of elements can be compared). If there is a real number m, let

X & gt=m, and x belongs to A (1).

Then m is called the lower bound of a, and the maximum m that satisfies (1) is called the lower supremum of a.

3) Any sequence can choose a convergent subsequence. In all the convergent subsequences of sequence ak, the maximum limit is called the upper limit of sequence ak.