X ∈ M₂ ⇒ [tex] \left[\begin{array}{ccc}m&n\\an&m+bn\end{array}\right] [/tex]
A · X = [tex] \left[\begin{array}{ccc}an&m+bn\\a(m+bn)&an+b(m+bn)\end{array}\right] [/tex], m, n ∈ Z (1)
X · A = [tex] \left[\begin{array}{ccc}an&m+bn\\a(m+bn)&an+b(m+bn)\end{array}\right] [/tex] , m, n ∈ Z (2)
Din (1) si (2) ⇒ A ·X = X·A, m,n ∈ Z.
