- Regular chain
In
computer algebra , a regular chain is a particular kind of triangular set in a multivariate polynomial ring over a field. It enhances the notion ofcharacteristic set .Introduction
Given a linear system, one can convert it to a triangular system via
Gaussian elimination . For the non-linear case, given a polynomial system F over a field, one can convert (decompose or triangularize) it to a finite set of triangular sets, in the sense that thealgebraic variety "V"(F) is described by these triangular sets. A triangular set may merely describe the empty set. To fix this degenerated case, the notion of regular chain was introduced, independently by Kalkbrener (1993), Yang and Zhang (1994). Regular chains also appear in Chou and Gao (1992). Regular chains are special triangular sets which are used in different algorithms for computing unmixed-dimensional decompositions of algebraic varieties. Without using factorization, these decompositions have better properties that the ones produced byWu's algorithm . Kalkbrener's original definition was based on the following observation: every irreducible variety is uniquely determined by one of itsgeneric points and varieties can be represented by describing the generic points of their irreducible components. These generic points are given by regular chains.Examples
Denote Q the rational number field. In Q [x1, x2, x3] with variable ordering x1 < x2 < x3,: is a triangular set and also a regular chain. Two generic points given by "T" are (a, a, a) and (a, -a, a) where "a" is transcendental over Q.Thus there are two irreducible components, given by { x2 - x1, x3 - x1 } and { x2 + x1, x3 - x1 }, repectively.Note that: (1) the
content of the second polynomial is x2, which does not contribute the generic points represented and thus can be reomved; (2) thedimension of each component is 1, the number of free variables in the regular chain.Formal definitions
The variables in the polynomial ring are always sorted as . A non-constant polynomial can be seen as a univariate polynomial in its greatest variable.
*Notions
The greatest variable in "f" is called its main variable, denoted by mvar(f). Let "u" be the main variable of "f" and write it as , where "e" is the degree of "f" w.r.t. "u" and is the leading coefficient of "f" w.r.t. "u". Then the initial of "f" is and "e" is its main degree.
*Triangular set
A non-empty subset "T" of is a triangular set, if the polynomials in "T" are non-constant and have distinct main variables. Hence, a triangular set is finite, and has cardinality at most "n".
*Regular chain
Let be a triangular set such that , be the initial of "t"i and "h" be the product of hi's. Then "T" is a regular chain if : , where each
resultant is computed with respect to the main variable of "t"i, respectively. This definition is from Yang and Zhang, which is of much algorithmic flavor.*Quasi-component and saturated ideal of a regular chain
The quasi-component "W"("T") described by the regular chain "T" is :, that is,the set difference of the varieties "V"("T") and "V"("h"). The attached algebraic object of a regular chain is its saturated ideal :. A classic result is that the
Zariski closure of "W"("T") equals the variety defined by sat("T"), that is,:,and its dimension is n - |T|, the difference of the number of variables and the number of polynomials in "T".*Triangular decompositions
In general, there are two ways to decompose a polynomial system "F". The first one is to decompose lazily, that is, only to represent its
generic points in the (Kalkbrener) sense,: .The second is to describe all zeroes in the (Lazard) sense,: .There are various algorithms available for triangular decompositions in either sense.Properties
Let "T" be a regular chain in the polynomial ring .
* The saturated ideal sat("T") is an unmixed ideal with dimension n − |"T"|.
* A regular chain holds a strong elimination property in the sense that:: .
* A polynomial "p" is in sat("T") if and only if p is pseudo-reduced to zero by "T", that is,: .:Hence the membership test for sat("T") is algorithmic.
* A polynomial p is a
zero-divisor modulo sat("T") if and only if and .:Hence the reguarity test for sat("T") is algorithmic.* Given a prime ideal "P", there exists a regular chain "C" such that "P" = sat("C").
* A triangular set is a regular chain if and only if it is a Ritt
characteristic set of its saturated ideal.See also
*
Characteristic set
*Groebner basis Further references
* P. Aubry, D. Lazard, M. Moreno Maza. On the theories of triangular sets. Journal of Symbolic Computation, 28(1-2):105-124, 1999.
* F. Boulier and F. Lemaire and M. Moreno Maza. Well known theorems on triangular systems and the D5 principle. Transgressive Computing 2006, Granada, Spain.
* E. Hubert. Notes on triangular sets and triangulation-decomposition algorithms I: Polynomial systems. LNCS, volume 2630, Springer-Verlag Heidelberg.
* F. Lemaire and M. Moreno Maza and Y. Xie. The RegularChains library. Maple Conference 2005.
* M. Kalkbrener: Algorithmic Properties of Polynomial Rings. J. Symb. Comput. 26(5): 525-581 (1998).
* M. Kalkbrener: A Generalized Euclidean Algorithm for Computing Triangular Representations of Algebraic Varieties. J. Symb. Comput. 15(2): 143-167 (1993).
* D. Wang. Computing Triangular Systems and Regular Systems. Journal of Symbolic Computation 30(2) (2000) 221-236.
* Yang, L., Zhang, J. (1994). Searching dependency between algebraic equations: an algorithm applied to automated reasoning. Artificial Intel ligence in Mathematics, pp. 14715, Oxford University Press.
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