A is a matrix, I is identity matrix and there exists such n that $\displaystyle A^n=4A^3+3A^2+2A+I$ and i have to prove that A is invertible. so i think we have to find such B that $\displaystyle AB=I$, then $\displaystyle A(A^{n-1}-4A^2-3A-2)=I$ but obviously "-2" is bad here.. $\displaystyle AI=A$ so maybe $\displaystyle A(A^{n-1}-4A^2-3A-2)=AI(A^{n-1}-4A^2-3A-2)=A(A^{n-1}-4A^2-3A-2I)=I$ makes proof right?