# Singular

### D.2.4 grobcov_lib

Library:
grobcov.lib
Purpose:

"Groebner Cover for parametric ideals.", Comprehensive Groebner Systems, Groebner Cover, Canonical Forms, Parametric Polynomial Systems, Automatic Deduction of Geometric Theorems, Dynamic Geometry, Loci, Envelope, Constructible sets. See: A. Montes A, M. Wibmer, "Groebner Bases for Polynomial Systems with parameters", Journal of Symbolic Computation 45 (2010) 1391-1425. (https://www.mat.upc.edu//en/people/antonio.montes/).

Important:
Recently published book:

A. Montes. " The Groebner Cover":
Springer, Algorithms and Computation in Mathematics 27 (2019) ISSN 1431-1550
ISBN 978-3-030-03903-5
ISBN 978-3-030-03904-2 (e-Book)
Springer Nature Switzerland AG 2018

https://www.springer.com/gp/book/9783030039035

The book can also be used as a user manual of all
the routines included in this library.
It defines and proves all the theoretic results used in the library, and shows examples of all the routines. There are many previous papers related to the subject, and the book actualices all the contents.

Authors:
Antonio Montes (Universitat Politecnica de Catalunya), Hans Schoenemann (Technische Universitaet Kaiserslautern).

Overview:
In 2010, the library was designed to contain
Montes-Wibmer's algorithm for computing the
Canonical Groebner Cover of a parametric ideal.
The central routine is grobcov.
Given a parametric ideal, grobcov outputs
its Canonical Groebner Cover, consisting of a set
of triplets of (lpp, basis, segment). The basis
(after normalization) is the reduced Groebner basis for each point of the segment. The segments
are disjoint, locally closed and correspond to
constant lpp (leading power product) of the basis,
and are represented in canonical representation.
The segments cover the whole parameter space.
The output is canonical, it only depends on the
given parametric ideal and the monomial order,
because the segments have
different lpph of the homogenized system.
This is much more than a simple Comprehensive
Groebner System. The algorithm grobcov allows
options to solve partially the problem when the
whole automatic algorithm does not finish in
reasonable time. Its existence was proved for the
first time by Michael Wibmer "Groebner bases for
families of affine or projective schemes",
JSC, 42,803-834 (2007).

grobcov uses a first algorithm cgsdr that outputs a disjoint reduced Comprehensive Groebner System
with constant lpp. For this purpose, in this library, the implemented algorithm is Kapur-Sun-Wang
algorithm, because it is actually the most efficient algorithm known for this purpose.
D. Kapur, Y. Sun, and D.K. Wang "A New Algorithm
for Computing Comprehensive Groebner Systems".
Proceedings of ISSAC'2010, ACM Press, (2010), 29-36.

The library has evolved to include new applications of the Groebner Cover, and new theoretical developments have been done.

A routine locus has been included to compute
loci of points, and determining the taxonomy of the components.
Additional routines to transform the output to string (locusdg, locusto) are also included and used in the Dynamic Geometry software GeoGebra. They were
described in:
M.A. Abanades, F. Botana, A. Montes, T. Recio:
"An Algebraic Taxonomy for Locus Computation in
Dynamic Geometry".
Computer-Aided Design 56 (2014) 22-33.

Routines for determining the generalized envelope of a family of hypersurfaces (envelop, AssocTanToEnv,
FamElemsToEnvCompPoints) are also included.

It also includes procedures for
Automatic Deduction of Geometric Theorems (ADGT).

The actual version also includes a
routine (ConsLevels) for computing the canonical form of a constructible set, given as a union of locally closed sets. It determines the canonical
levels of a constructible set. It is described in:
J.M. Brunat, A. Montes, "Computing the canonical
representation of constructible sets".
Math. Comput. Sci. (2016) 19: 165-178.
A complementary routine Levels transforms the output of ConsLevels into the proper locally closed sets
forming the levels of the constructible.
Antoher complementary routine Grob1Levels
has been included to select the locally closed sets of the segments of the grobcov that correspond to basis different from 1, add them together and return
the canonical form of this constructible set.
More recently (2019) given two locally closed sets
in canonical form the new routine DifConsLCSets
determines a set of locally closed sets equivalent to the difference them. The description of the
routine is submitted to the Journal of Symbolic Computation. This routine can be also used internally by ADGT
with the option "neg",1 . With this option
DifConsLCSets is used for the negative

This version was finished on 1/2/2020,

Notations:
Before calling any routine of the library grobcov, the user must define the ideal Q[a][x], and all the input polynomials and ideals defined on it.
Internally the routines define and use also other
ideals: Q[a], Q[x,a] and so on.

Procedures:

 D.2.4.1 grobcov Is the basic routine giving the canonical Groebner Cover of the parametric ideal F. This routine accepts many options, that allow to obtain results even when the canonical computation does not finish in reasonable time. D.2.4.2 cgsdr Is the procedure for obtaining a first disjoint, reduced Comprehensive Groebner System that is used in grobcov, but can also be used independently if only a CGS is required. It is a more efficient routine than buildtree (the own routine of 2010 that is no more available). Now, Kapur-Sun-Wang (KSW) algorithm is used. D.2.4.3 pdivi Performs a pseudodivision of a parametric polynomial by a parametric ideal. D.2.4.4 pnormalf Reduces a parametric polynomial f over V(E) V(N). E is the null ideal and N the non-null ideal over the parameters. D.2.4.5 Crep Computes the canonical C-representation of V(N) V(M). It can be called in Q[a] or in Q[a][x], but the ideals N,M can only contain parameters of Q[a]. D.2.4.6 Prep Computes the canonical P-representation of V(N) V(M). It can be called in Q[a] or in Q[a][x], but the ideals N,M can only contain parameters of Q[a]. D.2.4.7 PtoCrep Starting from the canonical Prep of a locally closed set computes its Crep. D.2.4.8 extendpoly Given the generic representation f of an I-regular function F defined by poly f on V(p) V(q) it returns its full representation. D.2.4.9 extendGC When the grobcov of an ideal has been computed with the default option ("ext",0) and the explicit option ("rep",2) (which is not the default), then one can call extendGC(GC) (and options) to obtain the full representation of the bases. With the default option ("ext",0) only the generic representation of the bases is computed, and one can obtain the full representation using extendGC. D.2.4.10 locus Special routine for determining the geometrical locus of points verifying given conditions. Given a parametric ideal J in Q[x1,..,xn][u1,..,um] with parameters (x1,..,xn) and variables (u1,..,um), representing the system determining the n-dimensional locus with tracer point (x1,..,xn) verifying certain properties, one can apply locus to the system F, for obtaining the locus. locus provides all the components of the locus and determines their taxonomy, that can be: "Normal", "Special", "Accumulation", "Degenerate". The mover are the u-variables. The user can ventually restrict them to a subset of them for geometrical reasons but this can change the true taxonomy. D.2.4.11 locusdg Is a special routine that determines the "Relevant" components of the locus in dynamic geometry. It is to be called to the output of locus and selects from it the "Normal", and"Accumulation" components. D.2.4.12 envelop Special routine for determining the envelop of a family of hyper-surfaces F in Q[x1,..,xn][t1,..,tm] depending on an ideal of constraints C in Q[t1,..,tm]. It computes the locus of the envelop, and determines the different components as well as its taxonomy: "Normal", "Special", "Accumulation", "Degenerate". (See help for locus). D.2.4.13 locusto Transforms the output of locus, locusdg, envelop into a string that can be reed from different computational systems. D.2.4.14 stdlocus Simple procedure to determine the components of the locus, alternative to locus that uses only standard GB computation. Cannot determine the taxonomy of the irreducible components. D.2.4.15 AssocTanToEnv Having computed an envelop component E of a family of hyper-surfaces F, with constraints C, it returns the parameter values of the associated tangent hyper-surface of the family passing at one point of the envelop component E. D.2.4.16 FamElemsAtEnvCompPoints Having computed an envelop component E of a family of hyper-surfaces F, with constraints C, it returns the parameter values of all the hyper-surfaces of the family passing at one point of the envelop component E. D.2.4.17 discrim Determines the factorized discriminant of a degree 2 polynomial in the variable x. The polynomial can be defined on any ring where x is a variable. The polynomial f can depend on parameters and variables. D.2.4.18 WLemma Given an ideal F in Q[a][x] and an ideal A in Q[a], it returns the list (lpp,B,S) were B is the reduced Groebner basis of the specialized F over the segment S, subset of V(A) with top A, determined by Wibmer's Lemma. S is determined in P-representation (or optionally in C-representation). The basis is given by I-regular functions. D.2.4.19 WLCGS Given a parametric ideal F in Q[a][x] determines a CGS in full-representation using WLemma D.2.4.20 intersectpar Auxiliary routine. Given a list of ideals defined on K[a][x] it determines the intersection of all of them in K[x,a] D.2.4.21 ADGT Given 4 ideals H,T,H1,T1 in Q[a][x], corresponding to a problem of Automatic Deduction of Geometric Theorems, it determines the supplementary conditions over the parameters for the Proposition (H and not H1) => (T and not T1) to be a Theorem. If H1=1 then H1 is not considered, and analogously for T1. D.2.4.22 ConsLevels Given a list of locally closed sets, constructs the canonical representation of the levels of A an its complement. D.2.4.23 Levels Transforms the output of ConsLevels into the proper Levels of the constructible set. D.2.4.24 Grob1Levels From the output of grobcov, Grob1Levels selects the segments of G with basis different from 1 (having solutions), and determines the levels of the constructible set formed by them. D.2.4.25 DifConsLCSets given the canonical forms of the constructible sets A and B, A=[a1,a2,..,ak], B=[b1,b2,...,bj], DifConsLCSets returns a list of locally closed sets of the set A minus B, that can be transformed into the canonical form of A minus B applying ConsLevels.