# nLab Jacobian

Contents

This entry is about the concept in differential geometry. For the concept of Jacobian variety see there.

### Context

#### Differential geometry

synthetic differential geometry

Introductions

from point-set topology to differentiable manifolds

Differentials

V-manifolds

smooth space

Tangency

The magic algebraic facts

Theorems

Axiomatics

cohesion

• (shape modality $\dashv$ flat modality $\dashv$ sharp modality)

$(\esh \dashv \flat \dashv \sharp )$

• dR-shape modality$\dashv$ dR-flat modality

$\esh_{dR} \dashv \flat_{dR}$

tangent cohesion

differential cohesion

singular cohesion

$\array{ && id &\dashv& id \\ && \vee && \vee \\ &\stackrel{fermionic}{}& \rightrightarrows &\dashv& \rightsquigarrow & \stackrel{bosonic}{} \\ && \bot && \bot \\ &\stackrel{bosonic}{} & \rightsquigarrow &\dashv& \mathrm{R}\!\!\mathrm{h} & \stackrel{rheonomic}{} \\ && \vee && \vee \\ &\stackrel{reduced}{} & \Re &\dashv& \Im & \stackrel{infinitesimal}{} \\ && \bot && \bot \\ &\stackrel{infinitesimal}{}& \Im &\dashv& \& & \stackrel{\text{étale}}{} \\ && \vee && \vee \\ &\stackrel{cohesive}{}& \esh &\dashv& \flat & \stackrel{discrete}{} \\ && \bot && \bot \\ &\stackrel{discrete}{}& \flat &\dashv& \sharp & \stackrel{continuous}{} \\ && \vee && \vee \\ && \emptyset &\dashv& \ast }$

Models

Lie theory, ∞-Lie theory

differential equations, variational calculus

Chern-Weil theory, ∞-Chern-Weil theory

Cartan geometry (super, higher)

# Contents

## Definition

If $f : \mathbb{R}^n \to \mathbb{R}^m$ is a $C^1$-differentiable map, between Cartesian spaces, its Jacobian matrix is the $(m \times n)$ matrix

$J(f) \in Mat_{m \times n}(C^0(\mathbb{R}, \mathbb{R}))$
$J(f)^i_j := \frac{\partial f^i}{\partial x^j},\,\,\,\,\,\,\,i=1,\ldots,m; j = 1,\ldots,n,$

where $x = (x^1,\ldots,x^n)$. Here the convention is that the upper index is a row index and the lower index is the column index; in particular $\mathbf{R}^n$ is the space of real column vectors of length $n$.

In more general situation, if $f = (f^1(x),\ldots,f^m(x))$ is differentiable at a point $x$ (and possibly defined only in a neighborhood of $x$), we define the Jacobian $J_p f$ of map $f$ at point $x$ as a matrix with real values $(J_p f)^i_j = \frac{\partial f^i}{\partial x^j}|_x$.

That is, the Jacobian is the matrix which describes the total derivative.

If $n=m$ the Jacobian matrix is a square matrix, hence its determinant $det(J(f))$ is defined and called the Jacobian of $f$ (possibly only at a point). Sometimes one refers to Jacobian matrix rather ambigously by Jacobian as well.

## Properties

The chain rule may be phrased by saying that the Jacobian matrix of the composition $\mathbf{R}^n\stackrel{f}\to\mathbf{R}^m\stackrel{g}\to\mathbf{R}^r$ is the matrix product of the Jacobian matrices of $g$ and of $f$ (at appropriate points).

If $g:M\to N$ is a $C^1$-map of $C^1$-manifolds, then the tangent map $T g: T M\to T N$ defined point by point abstractly by $(T_p g)(X_p)(f) = X_p(f\circ g)$, for $p\in M$, can in local coordinates be represented by a Jacobian matrix. Namely, if $(U,\phi)\ni p$ and $(V,\psi)\ni g(p)$ are charts and $X_p = \sum X^i\frac{\partial}{\partial x^i}|_p$ (i.e. $X_p(f) = \sum_i X^i_p \frac{\partial (f\circ \phi^{-1})}{\partial x^i}|_{\phi(p)}$ for all germs $f\in \mathcal{F}_p$), then

$(T_p g)(X_p) = \sum_{i,j} J_p(\psi \circ g\circ\phi^{-1})_i^j X^i_p \frac{\partial}{\partial y^j}|_{g(p)}$

Last revised on August 6, 2017 at 11:02:33. See the history of this page for a list of all contributions to it.