Does anyone how to prove the following completeness relations? $$\delta(\theta-\theta')=\frac{1}{\pi}\sum_{n\in\mathbb{Z}}\cos(n\theta)\cos(n\theta')$$ Is there an analogous relation for $\sin(\theta)$ as well?
2026-03-27 15:16:30.1774624590
Cosine function completeness relations
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