Trace

Suppose $A=(a_{ij})_{n\times n}$ is an square matrix. Trace of $A$ is the sum of the diagonal entries.

$$\text{trace}(A)=\text{Tr}(A)=\sum_{i=1}^n{a_{ii}}$$

Trace is also equal to the sum of eigenvalues.

$$\text{trace}(A)=\sum{\lambda_i}\text{ where } \lambda_i \in \text{spectrum of } A$$