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Counting fifth-order partials in ten variables

Question

Let f(x1,…,x10)f(x_1,\dots,x_{10}) be a real-valued function on R10\mathbb{R}^{10} with continuous partial derivatives of every order, so mixed partials do not depend on the order of differentiation. How many distinct partial derivatives of order 55 does ff have? (For instance, ∂5f/∂x35\partial^5 f/\partial x_3^5 and ∂5f/∂x1∂x42∂x92\partial^5 f/\partial x_1\partial x_4^2\partial x_9^2 each count as one.)

Short answer

20022002. A fifth-order partial is determined by how many times each variable is differentiated, i.e. by a non-negative solution of n1+⋯+n10=5n_1+\dots+n_{10}=5. Stars and bars gives (145)=2002\binom{14}{5}=2002.

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