I think it is solved with double counting. I tried to write unfolded, but I failed to solve the exercise. Can you help me?
2026-03-26 11:17:55.1774523875
How do I prove ${\binom{n}{0}}+{\binom{n}{3}}+{\binom{n}{6}}+\cdots ={\frac{1}{3}}\left(2^{n}+2\cos {\frac{n\pi }{3}}\right)?$
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that is a multisection of the binomial series