For the "if" argument, because T^n+dots+m_1T+m_0I is the zero matrix we have that I=(T^n+dots+m_1T)/(-m_0)=
Tcdot (T^{n-1}+dots+m_1I)/(-m_0) and so the matrix (-1/m_0)cdot (T^{n-1}+dots+m_1I) is the inverse of T. For "only if", suppose that m_0=0 (we put the n=1 case aside but it is easy) so that T^n+dots+m_1T=(T^{n-1}+dots+m_1I)T is the zero matrix. Note that T^{n-1}+dots+m_1I is not the zero matrix because the degree of the minimal polynomial is n . If T^{-1} exists then multiplying both (T^{n-1}+dots+m_1I)T and the zero matrix from the right by T^{-1} gives a contradiction.