Definition:Outer Product

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Definition

Given two vectors $\mathbf u = \tuple {u_1, u_2, \ldots, u_m}$ and $\mathbf v = \tuple {v_1, v_2, \ldots, v_n}$, their outer product $\mathbf u \otimes \mathbf v$ is defined as:

$\mathbf u \otimes \mathbf v = A = \begin{bmatrix}

u_1 v_1 & u_1 v_2 & \dots & u_1 v_n \\ u_2 v_1 & u_2 v_2 & \dots & u_2 v_n \\

\vdots & \vdots & \ddots & \vdots \\

u_m v_1 & u_m v_2 & \dots & u_m v_n \end{bmatrix}$

Index Notation

Given two vectors $u_i$ and $v_j$, their outer product $u_i \otimes v_j$ is defined as

$u_i \otimes v_j = a_{ij} = u_i v_j$

Matrix Multiplication

Given two vectors expressed as column matrices $\mathbf u$ and $\mathbf v$, their outer product $\mathbf u \otimes \mathbf v$ is defined as

$\mathbf u \otimes \mathbf v = A = \mathbf u \mathbf v^T$

Also known as

The outer product is sometimes referred to as the dyad product between two vectors.

Also see

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• $A \mathbf v = \mathbf u \norm {\mathbf v}^2$: see Vector Length.

• $\mathbf u \otimes \mathbf v = \paren {\mathbf v \otimes \mathbf u}^T$

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): outer product
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): outer product