In the setting of Metric Spaces: Balls, the Metric Topology, and the Real and Complex Numbers as Metric Spaces , let m , n , p , q ∈ N m,n,p,q\in\mathbb{N} m , n , p , q ∈ N , A , A ′ ∈ M m × n ( R ) A,A'\in\mathcal{M}_{m\times n}(\mathbb{R}) A , A ′ ∈ M m × n ( R ) , B , B ′ ∈ M n × p ( R ) B,B'\in\mathcal{M}_{n\times p}(\mathbb{R}) B , B ′ ∈ M n × p ( R ) , C ∈ M p × q ( R ) C\in\mathcal{M}_{p\times q}(\mathbb{R}) C ∈ M p × q ( R ) , s ∈ R s\in\mathbb{R} s ∈ R , x , y ∈ R n x,y\in\mathbb{R}^{n} x , y ∈ R n , z ∈ R p z\in\mathbb{R}^{p} z ∈ R p and w ∈ R m w\in\mathbb{R}^{m} w ∈ R m , with real matrices , their entries , entrywise operations , the matrix product , the matrix-vector product , the transpose , the identity matrices I m I_{m} I m , I n I_{n} I n and the standard basis vectors e k e_{k} e k of R n \mathbb{R}^{n} R n , the operations and origin 0 0 0 of R n \mathbb{R}^{n} R n and R m \mathbb{R}^{m} R m , the dot product , the Euclidean norm ∥ ⋅ ∥ \lVert\cdot\rVert ∥ ⋅ ∥ , and sums and squares as in Commutative Rings, Fields and Ordered Fields: Standard Notation §rings .
¶ ( A B ) C = A ( B C ) (AB)C=A(BC) ( A B ) C = A ( BC ) .
¶ A ( B + B ′ ) = A B + A B ′ A(B+B')=AB+AB' A ( B + B ′ ) = A B + A B ′ and ( A + A ′ ) B = A B + A ′ B (A+A')B=AB+A'B ( A + A ′ ) B = A B + A ′ B .
¶ ( s A ) B = s ( A B ) = A ( s B ) (sA)B=s(AB)=A(sB) ( s A ) B = s ( A B ) = A ( s B ) .
¶ I m A = A = A I n I_{m}A=A=AI_{n} I m A = A = A I n .
¶ A ( x + y ) = A x + A y A(x+y)=Ax+Ay A ( x + y ) = A x + A y , A ( x − y ) = A x − A y A(x-y)=Ax-Ay A ( x − y ) = A x − A y , A ( s x ) = s ( A x ) A(sx)=s(Ax) A ( s x ) = s ( A x ) , ( A + A ′ ) x = A x + A ′ x (A+A')x=Ax+A'x ( A + A ′ ) x = A x + A ′ x , ( s A ) x = s ( A x ) (sA)x=s(Ax) ( s A ) x = s ( A x ) , ( A B ) z = A ( B z ) (AB)z=A(Bz) ( A B ) z = A ( B z ) , I n x = x I_{n}x=x I n x = x and A 0 = 0 A0=0 A 0 = 0 .
¶ For every k ∈ [ n ] k\in[n] k ∈ [ n ] , ( A e k ) i = A i k (Ae_{k})_{i}=A_{ik} ( A e k ) i = A ik for all i ∈ [ m ] i\in[m] i ∈ [ m ] .
¶ ( A ⊤ ) ⊤ = A (A^{\top})^{\top}=A ( A ⊤ ) ⊤ = A , ( A + A ′ ) ⊤ = A ⊤ + A ′ ⊤ (A+A')^{\top}=A^{\top}+A'^{\top} ( A + A ′ ) ⊤ = A ⊤ + A ′ ⊤ , ( s A ) ⊤ = s A ⊤ (sA)^{\top}=sA^{\top} ( s A ) ⊤ = s A ⊤ , ( A B ) ⊤ = B ⊤ A ⊤ (AB)^{\top}=B^{\top}A^{\top} ( A B ) ⊤ = B ⊤ A ⊤ and I n ⊤ = I n I_{n}^{\top}=I_{n} I n ⊤ = I n .
¶ ( A x ) ⋅ w = x ⋅ ( A ⊤ w ) (Ax)\cdot w=x\cdot(A^{\top}w) ( A x ) ⋅ w = x ⋅ ( A ⊤ w ) .
¶ ∥ A x ∥ 2 ≤ ( ∑ i = 1 m ∑ j = 1 n A i j 2 ) ∥ x ∥ 2 \lVert Ax\rVert^{2}\le\Bigl(\sum_{i=1}^{m}\sum_{j=1}^{n}A_{ij}^{2}\Bigr)\lVert x\rVert^{2} ∥ A x ∥ 2 ≤ ( ∑ i = 1 m ∑ j = 1 n A ij 2 ) ∥ x ∥ 2 .