 Character of a tensor product

For finite-dimensional group representations, the character of their tensor product is the pointwise product of their characters. In a tensor-product basis, the diagonal entries of $A\otimes B$ are $A_{ii}{B}_{jj}$, so their sum is $(\mathrm{tr}A)(\mathrm{tr}B)$. This gives a direct route from a character table to tensor-product decompositions.