# Ramification

Ramification

In mathematics, ramification is a geometric term used for 'branching out', in the way that the square root function, for complex numbers, can be seen to have two "branches" differing in sign. It is also used from the opposite perspective (branches coming together) as when a covering map degenerates at a point of a space, with some collapsing together of the fibers of the mapping.

In complex analysis

In complex analysis, the basic model can be taken as the "z" $o$ "z""n" mapping in the complex plane, near "z" = 0. This is the standard local picture in Riemann surface theory, of ramification of order "n". It occurs for example in the Riemann-Hurwitz formula for the effect of mappings on the genus. See also branch point.

In algebraic topology

In a covering map the Euler-Poincaré characteristic should multiply by the number of sheets; ramification can therefore be detected by some dropping from that. The "z" $o$ "z""n" mapping shows this as a local pattern: if we exclude 0, looking at 0 < |z| < 1 say, we have (from the homotopy point of view) the circle mapped to itself by the "n"-th power map (Euler-Poincaré characteristic 0), but with the whole disk the Euler-Poincaré characteristic is 1, "n"-1 being the 'lost' points as the "n" sheets come together at "z" = 0.

In geometric terms, ramification is something that happens in "codimension two" (like knot theory, and monodromy); since "real" codimension two is "complex" codimension one, the local complex example sets the pattern for higher-dimensional complex manifolds. In complex analysis, sheets can't simply fold over along a line (one variable), or codimension one subspace in the general case. The ramification set (branch locus on the base, double point set above) will be two real dimensions lower than the ambient manifold, and so will not separate it into two 'sides', locally - there will be paths that trace round the branch locus, just as in the example. In algebraic geometry over any field, by analogy, it also happens in algebraic codimension one.

In algebraic number theory

Ramification in algebraic number theory means prime numbers factorising into some repeated prime ideal factors. Let "R" be the ring of integers of an algebraic number field "K" and "P" a prime ideal of "R". For each extension field "L" of "K" we can consider the integral closure "S" of "R" in "L" and the ideal "PS" of "S". This may or may not be prime, but assuming ["L":"K"] is finite it is a product of prime ideals

:"P"1"e"(1) ... "P""k""e"("k")

where the "P""i" are distinct prime ideals of "S". Then "P" is said to ramify in "L" if "e"("i") > 1 for some "i". In other words, "P" ramifies in "L" if the ramification index "e"("i") is greater than one for any "P""i". An equivalent condition is that "S"/"PS" has a non-zero nilpotent element - is not a product of finite fields. The analogy with the Riemann surface case was already pointed out by Dedekind and Heinrich M. Weber in the nineteenth century.

The ramification is tame when the ramification indices "e"("i") are all relatively prime to the residue characteristic "p" of P, otherwise wild. This condition is important in Galois module theory.

In local fields

The more detailed analysis of ramification in number fields can be carried out using extensions of the p-adic numbers, because it is a "local" question. In that case a quantitative measure of ramification is defined for Galois extensions, basically by asking how far the Galois group moves field elements with respect to the metric. A sequence of ramification groups is defined, reifying (amongst other things) "wild" (non-tame) ramification. This goes beyond the geometric analogue.

In algebraic geometry

There is also corresponding notion of unramified morphism in algebraic geometry. It serves to define étale morphisms.

* Eisenstein polynomial
* Newton polygon
* Puiseux expansion
* Branched covering

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• ramification — [ ramifikasjɔ̃ ] n. f. • 1541 anat.; lat. sav. ramificatio 1 ♦ Division en plusieurs rameaux; chacune des divisions ou des rameaux eux mêmes. Ramification d une tige à fleurs. ⇒ inflorescence. Spécialt Ramification des racines, des nervures des… …   Encyclopédie Universelle

• Ramification — Ram i*fi*ca tion (r[a^]m [i^]*f[i^]*k[=a] sh[u^]n), n. [Cf. F. ramification. See {Ramify}.] 1. The process of branching, or the development of branches or offshoots from a stem; also, the mode of their arrangement. [1913 Webster] 2. A small… …   The Collaborative International Dictionary of English

• ramification — (n.) 1670s, a branching out, from Fr. ramification, from ramifier (see RAMIFY (Cf. ramify)). Transferred sense of outgrowth, consequence first recorded 1755. Related: Ramifications …   Etymology dictionary

• ramification — Ramification. s. f. Terme d anatomie. Distribution d une grosse veine ou artere en plusieurs moindres qui en sont comme les rameaux. La ramification des veines …   Dictionnaire de l'Académie française

• ramification — index organ Burton s Legal Thesaurus. William C. Burton. 2006 …   Law dictionary

• ramification — [n] consequence, development bifurcation, branch, branching, breaking, complication, consequence, divarication, division, excrescence, extension, forking, offshoot, outgrowth, partition, radiation, result, sequel, subdividing, subdivision,… …   New thesaurus

• ramification — [ram΄ə fi kā′shən] n. [MFr < pp. of ML ramificare] 1. a ramifying or being ramified; specif., the arrangement of branches or offshoots, as on a plant 2. the result of ramifying; specif., a) a branch or offshoot b) a derived effect, consequence …   English World dictionary

• Ramification — En mathématiques, la ramification est un terme géométrique utilisé au sens de embranchement extérieur, à la façon dont la fonction racine carrée, pour les nombres complexes, peut être vue lorsqu on considère ses deux branches opposées. Il est… …   Wikipédia en Français

• RAMIFICATION — s. f. Production de rameaux, disposition des branches. Il est peu usité en ce sens.  Il se dit, par extension, en termes d Anatomie, de La division, de la distribution d une grosse veine, d une artère, d un nerf, en plusieurs moindres veines, etc …   Dictionnaire de l'Academie Francaise, 7eme edition (1835)

• ramification — noun /ɹamɪfɪˈkeɪʃən,ˌɹæməfəˈkeɪʃən/ a) A branching out, the act or result of developing branches; specifically the divergence of the stem and limbs of a plant into smaller ones, or of similar developments in blood vessels, anatomical structures… …   Wiktionary