Ramification (mathematics)
In
In complex analysis
In complex analysis, the basic model can be taken as the z → zn 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.
In algebraic topology
In a covering map the
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
In algebraic extensions of the rational numbers
Ramification in algebraic number theory means a prime ideal factoring in an extension so as to give some repeated prime ideal factors. Namely, let be the ring of integers of an algebraic number field , and a prime ideal of . For a field extension we can consider the ring of integers (which is the
where the are distinct prime ideals of . Then is said to ramify in if for some ; otherwise it is unramified. In other words, ramifies in if the ramification index is greater than one for some . An equivalent condition is that has a non-zero
The ramification is encoded in by the
The ramification is tame when the ramification indices are all relatively prime to the residue characteristic p of , otherwise wild. This condition is important in Galois module theory. A finite generically étale extension of Dedekind domains is tame if and only if the trace is surjective.
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 algebra
In
In algebraic geometry
There is also corresponding notion of
Let be a morphism of schemes. The support of the quasicoherent sheaf is called the ramification locus of and the image of the ramification locus, , is called the branch locus of . If we say that is formally unramified and if is also of locally finite presentation we say that is unramified (see Vakil 2017).
See also
- Eisenstein polynomial
- Newton polygon
- Puiseux expansion
- Branched covering
References
- Zbl 0956.11021.
- Vakil, Ravi (18 November 2017). The Rising Sea: Foundations of algebraic geometry (PDF). Retrieved 5 June 2019.