I found a formula from that can simply derive the entries the of inverse of Vandermonde matrix. However, there is one notation that I couldn't understand which I hope someone can help me or give me a simple example. Link: https://proofwiki.org/wiki/Inverse_of_Vandermonde_Matrix. I've never seen such a sigma notation. So, can someone explains what it is?
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1The subscript of the sigma describes a subset of indices satisfying some conditions, and the sum is over all such indices. It should become clear if you follow the proof; this is basically a restatement in matrix form of Lagrange interpolation. – Qiaochu Yuan Oct 13 '20 at 03:41