Prove that $ \frac{1}{2\pi i}\int_{C_r}\frac{e^{\lambda t}}{\lambda^{k+1}}d\lambda =\frac{t^k}{k!}$

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Let $C_r$ be the circle centered on $0$ with radius $r$ and $t\in \mathbb{R}$. How to show that $$ \frac{1}{2\pi i}\int_{C_r}\frac{e^{\lambda t}}{\lambda^{k+1}}d\lambda =\frac{t^k}{k!}$$

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A few possible methods:

  1. Apply Cauchy's integral formula

  2. Put $\displaystyle e^{\lambda t} = \sum\limits_{n=0}^\infty \frac{t^n}{n!} \lambda^n$ and integrate term-wise.

  3. Integrate by parts. (Leads to induction on $k$)

  4. Differentiate with respect to $t$.