
D.4.19 pointid_lib
 Library:
 pointid.lib
 Purpose:
 Procedures for computing a factorized lex GB of
the vanishing ideal of a set of points via the
AxisofEvil Theorem (M.G. Marinari, T. Mora)
 Author:
 Stefan Steidel, steidel@mathematik.unikl.de
 Overview:
 The algorithm of CerliencoMureddu [Marinari M.G., Mora T., A remark on a
remark by Macaulay or Enhancing Lazard Structural Theorem. Bull. of the
Iranian Math. Soc., 29 (2003), 103145] associates to each ordered set of
points A:={a1,...,as} in K^n, ai:=(ai1,...,ain)
 a set of monomials N and
 a bijection phi: A > N.
Here I(A):={f in K[x(1),...,x(n)]  f(ai)=0, for all 1<=i<=s} denotes the
vanishing ideal of A and N = Mon(x(1),...,x(n)) {LM(f)f in I(A)} is the
set of monomials which do not lie in the leading ideal of I(A) (w.r.t. the
lexicographical ordering with x(n)>...>x(1)). N is also called the set of
nonmonomials of I(A). NOTE: #A = #N and N is a monomial basis of
K[x(1..n)]/I(A). In particular, this allows to deduce the set of
cornermonomials, i.e. the minimal basis M:={m1,...,mr}, m1<...<mr, of its
associated monomial ideal M(I(A)), such that
M(I(A))= {k*mi  k in Mon(x(1),...,x(n)), mi in M},
and (by interpolation) the unique reduced lexicographical Groebner basis
G := {f1,...,fr} such that LM(fi)=mi for each i, that is, I(A)=<G>.
Moreover, a variation of this algorithm allows to deduce a canonical linear
factorization of each element of such a Groebner basis in the sense ot the
AxisofEvil Theorem by M.G. Marinari and T. Mora. More precisely, a
combinatorial algorithm and interpolation allow to deduce polynomials
y_mdi = x(m)  g_mdi(x(1),...,x(m1)),
i=1,...,r; m=1,...,n; d in a finite indexset F, satisfying
fi = (product of y_mdi) modulo (f1,...,f(i1))
where the product runs over all m=1,...,n; and all d in F.
Procedures:
