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$0_{m \times n}$  denotes the  $m \times n$  zero matrix , with all entries zero. $I_{n}$  denotes the $n \times n$  identity matrix , with $I_{ij}= \cases{ 1 & i = j \cr 0 & i $\ne$ j }$ For example;  $0_{2 \times 3} = \left\lbrack \matrix{ 0 & 0 & 0 \cr 0 & 0 & 0 } \right\rbrack,$  $I_{2} =  \left\lbrack \matrix{ 1 & 0 \cr 0 & 1 } \right\rbrack$ Transpose of Matrix  $A = \left\lbrack \matrix{ a & b & c \cr d & e & f } \right\rbrack {⇒}$  ${A^T} = \left\lbrack \matrix{ a & d \cr b & e \cr c & f } \right\rbrack$ Summation of Matrices (entrywise) $\left\lbrack \matrix{ a & b \cr c & d } \right\rbrack + \left\lbrack \matrix{ x & y \cr z & t} \right\rbrack = \left\lbrack \matrix{ a+x & b+y \cr c+z & d+t } \right\rbrack$ Sub...