# Definition:Cauchy Matrix

## Definition

The **Cauchy matrix**, commonly denoted $C_n$, can be found defined in two forms.

The **Cauchy matrix** is an $m \times n$ matrix whose elements are in the form:

- either $a_{ij} = \dfrac 1 {x_i + y_j}$
- or $a_{ij} = \dfrac 1 {x_i - y_j}$.

where $x_1, x_2, \ldots, x_m$ and $y_1, y_2, \ldots, y_n$ are elements of a field $F$.

They are of course equivalent, by taking $y'_j = -y_j$.

Note that $x_i + y_j$ (or $x_i - y_j$, depending on how the matrix is defined) must **not** be zero, or the element will be undefined.

Thus, writing the matrix out in full:

- $C_n := \begin{bmatrix} \dfrac 1 {x_1 + y_1} & \dfrac 1 {x_1 + y_2 } & \cdots & \dfrac 1 {x_1 + y_n} \\ \dfrac 1 {x_2 + y_1} & \dfrac 1 {x_2 + y_2 } & \cdots & \dfrac 1 {x_2 + y_n} \\ \vdots & \vdots & \ddots & \vdots \\ \dfrac 1 {x_m + y_1} & \dfrac 1 {x_m + y_2 } & \cdots & \dfrac 1 {x_m + y_n} \\ \end{bmatrix}$

or:

- $C_n := \begin{bmatrix} \dfrac 1 {x_1 - y_1} & \dfrac 1 {x_1 - y_2 } & \cdots & \dfrac 1 {x_1 - y_n} \\ \dfrac 1 {x_2 - y_1} & \dfrac 1 {x_2 - y_2 } & \cdots & \dfrac 1 {x_2 - y_n} \\ \vdots & \vdots & \ddots & \vdots \\ \dfrac 1 {x_m - y_1} & \dfrac 1 {x_m - y_2 } & \cdots & \dfrac 1 {x_m - y_n} \\ \end{bmatrix}$

## Also defined as

Some sources insist that:

- the elements $x_1, x_2, \ldots, x_m$ are all distinct
- the elements $y_1, y_2, \ldots, y_n$ are also all distinct.

If this is not the case, then its determinant is undefined.

## Also known as

Some sources report this as **Cauchy's matrix**.

## Source of Name

This entry was named for Augustin Louis Cauchy.

## Sources

- 1997: Donald E. Knuth:
*The Art of Computer Programming: Volume 1: Fundamental Algorithms*(3rd ed.) ... (previous) ... (next): $\S 1.2.3$: Sums and Products: Exercises -- Second Set