Past exam of the mathematics course of the University of Cambridge / 2016 / ib / Paper 1 / 9F / c

To prove that trace is the unique normalized cyclic linear functional, let $E_{ij}$ be the matrix units. For $i\ne j$, $E_{ii}{E}_{ij}=E_{ij}$ whereas $E_{ij}{E}_{ii}=0$, so the assumed identity for the linear functional gives $f(E_{ij})=0$. Also, $E_{ij}{E}_{ji}=E_{ii}$ and $E_{ji}{E}_{ij}=E_{jj}$, giving $f(E_{ii})=f(E_{jj})$. Write this common value as $c$. Since the matrix units form a basis,For $n=1$ the same conclusion follows immediately from the one-dimensional basis $E_{11}$. Finally, $f(I)=nc$; if this equals $n$, then $c=1$ over either specified field. The normalization makes $f$ exactly the trace.