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Power Series question
For example, in the formula $(x^0 + x^1 + x^2 + \cdots + x^9)^5$, what is the coefficient of $x^{23}$?
More generally, what is the coefficient of $x^i$ where $0\leq i\leq nk$ ?
I'm really interested in how you get an answer. Do you simply count? Do you solve it with generating functions? Even dynamic programming?