Exercise 7.2

Exercise 2: If { f n } and { g n } converge uniformly on a set E , prove that { f n + g n } converges uniformly on E . If, in addition, { f n } and { g n } are sequences of bounded functions, prove that { f n g n } converges uniformly on E .

Answers

Let 𝜀 > 0 . By Theorem 7.8 there is an integer N such that for all n , m N and x E ,

| f n ( x ) f m ( x ) | < 𝜀 2 | g n ( x ) g m ( x ) | < 𝜀 2 .

Hence for all n , m N and x E ,

| ( f n + g n ) ( x ) ( f m + g m ) ( x ) | | f n ( x ) f m ( x ) | + | g n ( x ) g m ( x ) | < 𝜀 .

Hence, also by Theorem 7.8, { f n + g n } converges uniformly on E .

If { f n } and { g n } are uniformly convergent sequences of bounded functions, then by Exercise 1 they are uniformly bounded. That is, there is a number M such that | f n ( x ) | < M and | g n ( x ) | < M for all n and for all x E . By Theorem 7.8 there is an integer N such that for all n N and for all x E we have

| f n ( x ) f m ( x ) | < 𝜀 M | g n ( x ) g m ( x ) | < 𝜀 M .

Hence for all n , m N and x E ,

| f n ( x ) g n ( x ) f m ( x ) g m ( x ) | | f n ( x ) g n ( x ) f m ( x ) g n ( x ) | + | f m ( x ) g n ( x ) f m ( x ) g m ( x ) | M | f n ( x ) f m ( x ) | + M | g n ( x ) g m ( x ) | 𝜀

Hence, also by Theorem 7.8, { f n g n } converges uniformly on E .

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2023-08-07 00:00
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