Trace of Matrix Product
Jump to navigation
Jump to search
Theorem
Let $\mathbf A$ and $\mathbf B$ be square matrices of order $n$.
Let $\mathbf A \mathbf B$ be the (conventional) matrix product of $\mathbf A$ and $\mathbf B$.
Then:
- $\ds \map \tr {\mathbf A \mathbf B} = \sum_{i \mathop = 1}^n \sum_{j \mathop = 1}^n a_{i j} b_{j i}$
where $\map \tr {\mathbf A \mathbf B}$ denotes the trace of $\mathbf A \mathbf B$.
Using the Einstein summation convention, this can be expressed as:
- $\map \tr {\mathbf A \mathbf B} = a_{i j} b_{j i}$
Proof
Let $\mathbf C := \mathbf A \mathbf B$.
By definition of matrix product:
- $\ds c_{i k} = \sum_{j \mathop = 1}^n a_{i j} b_{j k}$
Thus for the diagonal elements:
- $\ds c_{i i} = \sum_{j \mathop = 1}^n a_{i j} b_{j i}$
By definition of trace:
- $\ds \map \tr {\mathbf C} = \sum_{i \mathop = 1}^n c_{i i}$
Hence the result.
$\blacksquare$
Sources
- 1980: A.J.M. Spencer: Continuum Mechanics ... (previous) ... (next): $2.2$: The summation convention: $(2.18)$