Questions about indexing and tensor contractions

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Questions about indexing and tensor contractions

Boost - uBLAS mailing list
Hello, 

Recently, I came across the index utility "number_equal_indices<>" method, which is used in the implementation of tensor contraction. If my understanding is correct, this appears to facilitate the implementation of Einstein summations, where common indices between two tensors/matrices are summed. 

My question now is - what happens when there are more than one equal index (i.e. both tensors use indices i and j)?

Thanks,

Thomas Yang 

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