cotangent bundle

Given a manifold (or generalized smooth space) $X$, the **cotangent bundle** $T^*(X)$ of $X$ is the vector bundle over $X$ dual to the tangent bundle $T_*(X)$ of $X$. A **cotangent vector** or **covector** on $X$ is an element of $T^*(X)$. The **cotangent space** of $X$ at a point $a$ is the fiber $T^*_a(X)$ of $T^*(X)$ over $a$; it is a vector space. A **covector field** on $X$ is a section of $T^*(X)$. (More generally, a differential form on $X$ is a section of the exterior algebra of $T^*(X)$; a covector field is a **differential $1$-form**.)

Given a covector $\omega$ at $a$ and a tangent vector $v$ at $a$, the pairing $\langle{\omega,v}\rangle$ is a scalar (a real number, usually). This (with some details about linearity and universality) is basically what it means for $T^*(X)$ to be dual to $T_*(X)$. More globally, given a covector field $\omega$ and a tangent vector field $v$, the paring $\langle{\omega,v}\rangle$ is a scalar function on $X$.

Given a point $a$ in $X$ and a differentiable (real-valued) partial function $f$ defined near $a$, the **differential** $\mathrm{d}_a f$ of $f$ at $a$ is a covector on $X$ at $a$; given a tangent vector $v$ at $a$, the pairing is given by

$\langle{\mathrm{d}_a f, v}\rangle = v[f] ,$

thinking of $v$ as a derivation on differentiable functions defined near $a$. (It is really the germ at $a$ of $f$ that matters here.) More globally, given a differentiable function $f$, the **differential** $\mathrm{d}f$ of $f$ is a covector field on $X$; given a vector field $v$, the pairing is given by

$\langle{\mathrm{d}f, v}\rangle = v[f] ,$

thinking of $v$ as a derivation on differentiable functions.

One can also *define* covectors at $a$ to be germs of differentiable functions at $a$, modulo the equivalence relation that $\mathrm{d}_a f = \mathrm{d}_a g$ if $f - g$ is constant on some neighbourhood of $a$. In general, a covector field won't be of the form $\mathrm{d}f$, but it will be a sum of terms of the form $h \mathrm{d}f$. More specifically, a covector field $\omega$ on a coordinate patch can be written

$\omega = \sum_i \omega_i\, \mathrm{d}x^i$

in local coordinates $(x^1,\ldots,x^n)$. This fact can also be used as the basis of a definition of the cotangent bundle.

Revised on May 20, 2013 12:28:50
by Urs Schreiber
(89.204.130.66)