# Definition:Matrix/Summation Convention

## Definition

The summation convention is a notational device used in the manipulation of matrices, in particular square matrices in the context of physics and applied mathematics.

If the same index occurs twice in a given expression involving matrices, then summation over that index is automatically assumed.

Thus the summation sign can be omitted, and expressions can be written more compactly.

## Examples

### Trace of Matrix

The trace of $A$, using the summation convention, is:

$\map \tr A = a_{ii}$

### Substitution Rule

The Substitution Rule for Matrices can be expressed using the 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 $\left({j, k}\right)$ of $\mathbf A$.

The index which appears twice in these expressions is the element $j$, which is the one summated over.

### Determinant of Order 3

The determinant of a square matrix of order $3$ $\mathbf A$ can be expressed using the summation convention as:

$\map \det {\mathbf A} = \dfrac 1 6 \map \sgn {i, j, k} \map \sgn {r, s, t} a_{i r} a_{j s} a_{k t}$

Note that there are $6$ indices which appear twice, and so $6$ summations are assumed.

### Matrix Product

The matrix product of $\mathbf A$ and $\mathbf B$ can be expressed using the summation convention as:

Then:

$c_{i j} := a_{i k} \circ b_{k j}$

The index which appears twice in the expressions on the right hand side is the entry $k$, which is the one summated over.