rational function

Given an integral domain $R$, the commutative *ring of rational functions* $R(z)$ with coefficients in $R$ is the field of fractions of the polynomial ring $R[z]$.

Let $X$ be an affine variety over a field $k$ with the ring of regular function?s $\mathcal{O}(X)$. A **rational function** is any element of the field of fractions of $\mathcal{O}(X)$, that is the function field of the variety.

In either case, rational functions are equivalence classes of fractions; they need not be functions defined everywhere. If $k$ is a field, then to each fraction $\frac{p(x)}{q(x)} \in k(x)$ with $p, q \in k[x]$ relatively prime, we may associate a partial function $k \rightharpoonup k$ whose domain consists of $a \in k$ such that $q(a) \neq 0$ (defining $q(a)$ as usual as the value of $q$ under the unique $k$-algebra map $k[x] \to k$ sending $x$ to $a$), and sending each $a$ in the domain to $\frac{p(a)}{q(a)}$. For the purposes of most elementary mathematics, the domain given here may be described as the βnatural domainβ of the rational function.

It is perhaps more illuminating to think of this partial function (with domain $D$) as coming from a (total) function $[p/q]: \mathbb{P}^1(k) \to \mathbb{P}^1(k)$ on the projective line, where we have inclusion functions $i: k \to \mathbb{P}^1(k)$ and the partial function is given by the pair of projection maps in the pullback

$\array{
D & \to & k \\
\downarrow & & \downarrow_\mathrlap{i} \\
k & \underset{[p/q] \circ i}{\to} & \mathbb{P}^1(k)
}$

It should also be noticed that such endofunctions $[p/q]$ on $\mathbb{P}^1(k)$ are closed under composition (except that special provision must be made for the constant function valued at $\infty$, which corresponds to the βfractionβ $1/0$).

number fields (βfunction fields of curves over F1β) | function fields of curves over finite fields $\mathbb{F}_q$ (arithmetic curves) | Riemann surfaces/complex curves | |
---|---|---|---|

affine and projective line | |||

$\mathbb{Z}$ (integers) | $\mathbb{F}_q[z]$ (polynomials, function algebra on affine line $\mathbb{A}^1_{\mathbb{F}_q}$) | $\mathcal{O}_{\mathbb{C}}$ (holomorphic functions on complex plane) | |

$\mathbb{Q}$ (rational numbers) | $\mathbb{F}_q(z)$ (rational functions) | meromorphic functions on complex plane | |

$p$ (prime number/non-archimedean place) | $x \in \mathbb{F}_p$ | $x \in \mathbb{C}$ | |

$\infty$ (place at infinity) | $\infty$ | ||

$Spec(\mathbb{Z})$ (Spec(Z)) | $\mathbb{A}^1_{\mathbb{F}_q}$ (affine line) | complex plane | |

$Spec(\mathbb{Z}) \cup place_{\infty}$ | $\mathbb{P}_{\mathbb{F}_q}$ (projective line) | Riemann sphere | |

$\partial_p \coloneqq \frac{(-)^p - (-)}{p}$ (Fermat quotient) | $\frac{\partial}{\partial z}$ (coordinate derivation) | β | |

genus of the rational numbers = 0 | genus of the Riemann sphere = 0 | ||

formal neighbourhoods | |||

$\mathbb{Z}_p$ (p-adic integers) | $\mathbb{F}_q[ [ t -x ] ]$ (power series around $x$) | $\mathbb{C}[ [z-x] ]$ (holomorphic functions on formal disk around $x$) | |

$Spf(\mathbb{Z}_p)\underset{Spec(\mathbb{Z})}{\times} X$ (β$p$-arithmetic jet spaceβ of $X$ at $p$) | formal disks in $X$ | ||

$\mathbb{Q}_p$ (p-adic numbers) | $\mathbb{F}_q((z-x))$ (Laurent series around $x$) | $\mathbb{C}((z-x))$ (holomorphic functions on punctured formal disk around $x$) | |

$\mathbb{A}_{\mathbb{Q}} = \underset{p\; place}{\prod^\prime}\mathbb{Q}_p$ (ring of adeles) | $\mathbb{A}_{\mathbb{F}_q((t))}$ ( adeles of function field ) | $\underset{x \in \mathbb{C}}{\prod^\prime} \mathbb{C}((z-x))$ (restricted product of holomorphic functions on all punctured formal disks, finitely of which do not extend to the unpunctured disks) | |

$\mathbb{I}_{\mathbb{Q}} = GL_1(\mathbb{A}_{\mathbb{Q}})$ (group of ideles) | $\mathbb{I}_{\mathbb{F}_q((t))}$ ( ideles of function field ) | $\underset{x \in \mathbb{C}}{\prod^\prime} GL_1(\mathbb{C}((z-x)))$ | |

theta functions | |||

Jacobi theta function | |||

zeta functions | |||

Riemann zeta function | Goss zeta function | ||

branched covering curves | |||

$K$ a number field ($\mathbb{Q} \hookrightarrow K$ a possibly ramified finite dimensional field extension) | $K$ a function field of an algebraic curve $\Sigma$ over $\mathbb{F}_p$ | $K_\Sigma$ (sheaf of rational functions on complex curve $\Sigma$) | |

$\mathcal{O}_K$ (ring of integers) | $\mathcal{O}_{\Sigma}$ (structure sheaf) | ||

$Spec_{an}(\mathcal{O}_K) \to Spec(\mathbb{Z})$ (spectrum with archimedean places) | $\Sigma$ (arithmetic curve) | $\Sigma \to \mathbb{C}P^1$ (complex curve being branched cover of Riemann sphere) | |

$\frac{(-)^p - \Phi(-)}{p}$ (lift of Frobenius morphism/Lambda-ring structure) | $\frac{\partial}{\partial z}$ | β | |

genus of a number field | genus of an algebraic curve | genus of a surface | |

formal neighbourhoods | |||

$v$ prime ideal in ring of integers $\mathcal{O}_K$ | $x \in \Sigma$ | $x \in \Sigma$ | |

$K_v$ (formal completion at $v$) | $\mathbb{C}((z_x))$ (function algebra on punctured formal disk around $x$) | ||

$\mathcal{O}_{K_v}$ (ring of integers of formal completion) | $\mathbb{C}[ [ z_x ] ]$ (function algebra on formal disk around $x$) | ||

$\mathbb{A}_K$ (ring of adeles) | $\prod^\prime_{x\in \Sigma} \mathbb{C}((z_x))$ (restricted product of function rings on all punctured formal disks around all points in $\Sigma$) | ||

$\mathcal{O}$ | $\prod_{x\in \Sigma} \mathbb{C}[ [z_x] ]$ (function ring on all formal disks around all points in $\Sigma$) | ||

$\mathbb{I}_K = GL_1(\mathbb{A}_K)$ (group of ideles) | $\prod^\prime_{x\in \Sigma} GL_1(\mathbb{C}((z_x)))$ | ||

Galois theory | |||

Galois group | β | $\pi_1(\Sigma)$ fundamental group | |

Galois representation | β | flat connection (βlocal systemβ) on $\Sigma$ | |

class field theory | |||

class field theory | β | geometric class field theory | |

Hilbert reciprocity law | Artin reciprocity law | Weil reciprocity law | |

$GL_1(K)\backslash GL_1(\mathbb{A}_K)$ (idele class group) | β | ||

$GL_1(K)\backslash GL_1(\mathbb{A}_K)/GL_1(\mathcal{O})$ | β | $Bun_{GL_1}(\Sigma)$ (moduli stack of line bundles, by Weil uniformization theorem) | |

non-abelian class field theory and automorphy | |||

number field Langlands correspondence | function field Langlands correspondence | geometric Langlands correspondence | |

$GL_n(K) \backslash GL_n(\mathbb{A}_K)//GL_n(\mathcal{O})$ (constant sheaves on this stack form unramified automorphic representations) | β | $Bun_{GL_n(\mathbb{C})}(\Sigma)$ (moduli stack of bundles on the curve $\Sigma$, by Weil uniformization theorem) | |

Tamagawa-Weil for number fields | Tamagawa-Weil for function fields | ||

theta functions | |||

Hecke theta function | functional determinant line bundle of Dirac operator/chiral Laplace operator on $\Sigma$ | ||

zeta functions | |||

Dedekind zeta function | Weil zeta function | zeta function of a Riemann surface/of the Laplace operator on $\Sigma$ | |

higher dimensional spaces | |||

zeta functions | Hasse-Weil zeta function |

Revised on January 2, 2015 16:02:19
by Urs Schreiber
(127.0.0.1)