# coincident-root-loci: Equivariant CSM classes of coincident root loci

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This library contians a set of function to compute, among others, the GL(2)-equivariant Chern-Schwartz-MacPherson classes of coincident root loci, which are subvarieties of the space of unordered n-tuples of points in the complex projective line. To such an n-tuples we can associate a partition of n given by the multiplicities of the distinct points; this stratifies the set of all n-tuples, and we call these strata "coincident root loci". This package is supplementary software for a forthcoming paper.  #### Maintainer's Corner

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Versions [RSS] 0.2, 0.3 array (>=0.5), base (>=4 && <5), combinat (>=0.2.10.0), containers (>=0.5), polynomial-algebra (>=0.1), random, transformers [details] BSD-3-Clause (c) 2015-2021 Balazs Komuves Balazs Komuves bkomuves (plus) hackage (at) gmail (dot) com Math https://hub.darcs.net/bkomuves/coincident-root-loci head: darcs get https://hub.darcs.net/bkomuves/coincident-root-loci by BalazsKomuves at 2021-07-26T16:22:19Z NixOS:0.3 1 direct, 0 indirect [details] 992 total (6 in the last 30 days) 2.0 (votes: 1) [estimated by Bayesian average] λ λ λ Docs available Last success reported on 2021-07-28

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# Characteristic classes of coincident root loci

Coincident root loci (or discriminant strata) are subsets of the space of homogeneous polynomials in two variables defined by root multiplicities: A nonzero degree n polynomial has n roots in the complex projective line P^1, but some of these can coincide, which gives us a partition of n. Hence for each partition lambda we get a set of polynomials (those with root multiplicities given by lambda), which together stratify the space of these polynomials, which (modulo multiplying by scalars) is P^n. These are quasi-projective varieties, invariant under the action of GL(2); their closures are highly singular projective varieties, making them a good example for studying invariants of singular varieties.

This package contains a number of different algorithms to compute invariants and characteristic classes of these varieties:

• degree
• Euler characteristic
• the fundamental class in equivariant cohomology
• Chern-Schwartz-MacPherson (CSM) class, Segre-SM class
• equivariant CSM class
• Hirzebruch Chi-y genus
• Todd class, motivic Hirzebruch class
• motivic Chern class
• equivariant motivic Chern class

Another (better organized) Mathematica implementation is available at https://github.com/bkomuves/mathematica-packages.

# Example usage

For example if you want to know what is the equivariant CSM class of the (open) loci corresponding to the partition [2,2,1,1], you can use the following piece of code:

{-# LANGUAGE TypeApplications #-}

import Math.Combinat.Partitions
import Math.RootLoci.Algebra.SymmPoly ( AB )
import Math.Algebra.Polynomial.Pretty ( pretty )
import Math.RootLoci.CSM.Equivariant.Umbral

csm ps = umbralOpenCSM @AB (mkPartition ps)

main = do
putStrLn $pretty$ csm [2,2,1,1]