Jacobian

2020 Mathematics Subject Classification: Primary: 26B10 Secondary: 26B15 [MSN][ZBL]

## Contents

* 1 Jacobian Matrix
* 2 Jacobian determinant
* 3 Generalizations of the Jacobian determinant
* 4 Jacobian variety
* 5 References

#### Jacobian Matrix[edit]

Also called Jacobi matrix. Let $U\subset \mathbb R^n$, $f: U\to \mathbb R^m$ and assume that $f$ is differentiable at the point $y\in U$. The Jacobi matrix of $f$ at $y$ is then the matrix \begin{equation}\label{e:Jacobi_matrix} Df|_y := \left( \begin{array}{llll} \frac{\partial f^1}{\partial x_1} (y) & \frac{\partial f^1}{\partial x_2} (y)&\qquad \ldots \qquad & \frac{\partial f^1}{\partial x_n} (y)\\\ \frac{\partial f^2}{\partial x_1} (y) & \frac{\partial f^2}{\partial x_2} (y)&\qquad \ldots \qquad & \frac{\partial f^2}{\partial x_n} (y)\\\ \\\ \vdots & \vdots & &\vdots\\\ \\\ \frac{\partial f^m}{\partial x_1} (y) & \frac{\partial f^m}{\partial x_2} (y)&\qquad \ldots \qquad & \frac{\partial f^m}{\partial x_n} (y) \end{array}\right)\, , \end{equation} where $(f^1, \ldots, f^m)$ are the coordinate functions of $f$ and $x_1,\ldots, x_n$ denote the standard system of coordinates in $\mathbb R^n$.

#### Jacobian determinant[edit]

Also called Jacobi determinant. If $U$, $f$ and $y$ are as above and $m=n$, the Jacobian determinant of $f$ at $y$ is the determinant of the Jacobian matrix \ref{e:Jacobi_matrix}. Some authors use the same name for the absolute value of such determinant. If $U$ is an open set and $f$ a locally invertible $C^1$ map, the absolute value of the Jacobian determinant gives the infinitesimal dilatation of the volume element in passing from the variables $x_1, \ldots, x_n$ to the variables $f_1,\ldots, f_n$. Therefore the Jacobian determinant plays a crucial role when changing variables in integrals, see Sections 3.2 and 3.3 of [EG] (see also Differential form and Integration on manifolds).

#### Generalizations of the Jacobian determinant[edit]

The Jacobian determinant can be generalized also to the case where the dimension of the target differs from that of the domain (see Section 3.2 of [EG]). More precisely, let $f$, $U$, $n$, $m$ and $y$ be as above:

* If $m<n$, the Jacobian of $f$ at $y$ is given by the square root of the determinant of $Df_y\cdot (Df_y)^t$ (where $Df_y^t$ denotes the transpose of the matrix $Df_y$);
* If $m>n$, the Jacobian of $f$ at $y$ is given by the square root of the determinant of $(Df_y)^t\cdot Df_y$.

These generalizations play a key role respectively in the Coarea formula and Area formula.

An important characterization of the Jacobian is then given by the Cauchy Binet formula: $Jf(y)^2$ is the sum of the squares of the determinants of all $n\times n$ minors of $Df|_y$ (cp. with Theorem 4 in Section 3.2.1 of [EG]).

#### Jacobian variety[edit]

See Jacobi variety.

#### References[edit]

[EG] | L.C. Evans, R.F. Gariepy, "Measure theory and fine properties of functions" Studies in Advanced Mathematics. CRC Press, Boca Raton, FL, 1992. MR1158660 Zbl 0804.2800

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