Trace of Unit Matrix

Theorem
Let $\mathbf I_n$ be the identity matrix of order $n$.

Then:
 * $\operatorname{tr} \left({\mathbf I_n}\right) = n$

where $\operatorname{tr} \left({\mathbf I_n}\right)$ denotes the trace of $\mathbf I_n$.

Proof
By definition:
 * $\mathbf I_n := \left[{a}\right]_n: a_{i j} = \delta_{i j}$

That is: each of the elements on the main diagonal is equal to $1$.

There are $n$ such elements.

Hence the result.