Math basics › Linear algebra
Matrix multiplication
Row times column
Entry (i, j) of AB is row i of A dotted with column j of B: multiply matching entries and add.
Check the sizes
(m × n) times (n × p) gives m × p. The inside numbers must match, or the product does not exist.
Order matters
AB and BA are usually different, and one can exist without the other. Never swap them to make the algebra easier.