Dirichlet's Test for Uniform Convergence

Theorem
Suppose:


 * The partial sums of $$\sum_{n=1}^{\infty}a_n(x)$$ are uniformly bounded on $$D$$.


 * $${b_n(x)}$$ is monotonic for each $$x\in D$$.


 * $$b_n(x)\rightarrow 0$$ uniformly on $$D$$.

Then:

$$\sum_{n=1}^{\infty}a_n(x)b_n(x)$$ converges uniformly on $$D$$

Proof
Suppose $$b_n(x)\geq b_{n+1}(x)$$ for each $$x\in D$$.

All we need to show is that $$\sum_{n=1}^{\infty}|b_n(x)-b_{n+1}(x)|$$ converges uniformly on $$D$$.

To do this we show that the Cauchy Criterion holds.

Assign $$\epsilon<0$$, then $$\exist N \in\mathbb{N}$$ such that $$\forall x\in D$$ and $$\forall n\geq N, |b_n(x)|<\frac{\epsilon}{2}$$.

If $$x\in D$$ and $$n>m\geq N$$ then,

$$ $$ $$ $$ $$ $$