Demonstrate that a given set of matrices is multiplicatively closed


Denote by \mathcal{A} the set of all 2×2 matrices with real number entries. Let M = \left[ {1 \atop 0} {1 \atop 1} \right] and let \mathcal{B} = \{ X \in \mathcal{A} \ |\ MX = XM \}.
Prove that if P,Q \in \mathcal{B}, then PQ \in \mathcal{B}, where juxtaposition denotes the usual matrix product.

Recall that matrix multiplication is associative. Then we have (PQ)M = P(QM) = P(MQ) = (PM)Q = (MP)Q = M(PQ), and thus PQ \in \mathcal{B}\blacksquare





No comments:

Post a Comment