# Characterizing the trace of a matrix

I have been studying for my finals lately, and so I decided to put together a proof of a nice exercise I found in some book. The trace function, given by $$tr : \mathbb{K}^{n \times n} \to \mathbb{K}$$, is defined as tr(A) = \sum_{i=1}^n A_{ii} First of all, the proof of additivity $$\begin{split} tr(A + B) &= \sum_{i=1}^n (A+B)_{ii} \\ &= \sum_{i=1}^n (A)_{ii} + (B)_{ii} \\ &= \sum_{i=1}^n (A)_{ii} + \sum_{i=1}^n (B)_{ii} \\ &= tr(A) + tr(B) \end{split}$$ Afterwards, the proof of homogeneity