>>557

tr(AB)
=Σ[i]Σ[j]a(i,j)*b(j,i)
=Σ[i]Σ[j]b(j,i)*a(i,j)
=Σ[j]Σ[i]b(i,j)*a(j,i)
=Σ[i]Σ[j]b(i,j)*a(j,i)
=tr(BA)