Questions about indexing and tensor contractions

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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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<div dir=3D"ltr">Hello,=C2=A0<div><br></div><div>Recently, I came across th=
e index utility &quot;number_equal_indices&lt;&gt;&quot; method, which is u=
sed in the implementation of tensor contraction. If my understanding is cor=
rect, this appears to facilitate the implementation of Einstein summations,=
 where common indices between two tensors/matrices are summed.=C2=A0</div><=
div><br></div><div>My question now is - what happens when there are more th=
an one equal index (i.e. both tensors use indices i and j)?</div><div><br><=
/div><div>Thanks,</div><div><br></div><div>Thomas Yang=C2=A0</div></div>

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