Substitution Rule for Matrices/Einstein Summation Convention
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Summation Convention
Let $\mathbf A$ be a square matrix of order $n$.
The Substitution Rule for Matrices can be expressed using the Einstein summation convention as:
- $(1): \quad \delta_{i j} a_{j k} = a_{i k}$
- $(2): \quad \delta_{i j} a_{k j} = a_{k i}$
where:
- $\delta_{i j}$ is the Kronecker delta
- $a_{j k}$ is element $\tuple {j, k}$ of $\mathbf A$.
The index which appears twice in these expressions is the element $j$, which is the one summated over.
Sources
- 1980: A.J.M. Spencer: Continuum Mechanics ... (previous) ... (next): $2.2$: The summation convention