Can someone explain this matrix notation $A = [ A_1 A_2 A_3]$? For a matrix $A$, I know that $a_{12}$ or $A_{12}$ means row $1$, column $2$ in matrix $A$. However, what does $A_1$ refer to? For example, one reference noted this about a $3\times 3$ matrix like this: $A = [ A_1 A_2 A_3]$.
