IDEALS: Arithmetic for polynomial ideals
Date: 2020-10-07 18:10:00

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### 16.31 IDEALS: Arithmetic for polynomial ideals

This package implements the basic arithmetic for polynomial ideals by exploiting the Gröbner bases package of REDUCE. In order to save computing time all intermediate Gröbner bases are stored internally such that time consuming repetitions are inhibited.

Author: Herbert Melenk.

#### 16.31.1 Introduction

This package implements the basic arithmetic for polynomial ideals by exploiting the Gröbner bases package of REDUCE. In order to save computing time all intermediate Gröbner bases are stored internally such that time consuming repetitions are inhibited. A uniform setting facilitates the access.

#### 16.31.2 Initialization

Prior to any computation the set of variables has to be declared by calling the operator I_setting . E.g. in order to initiate computations in the polynomial ring Q[x,y,z] call

I_setting(x,y,z);

A subsequent call to I_setting allows one to select another set of variables; at the same time the internal data structures are cleared in order to free memory resources.

#### 16.31.3 Bases

An ideal is represented by a basis (set of polynomials) tagged with the symbol I, e.g.

u := I(x*z-y**2, x**3-y*z);

Alternatively a list of polynomials can be used as input basis; however, all arithmetic results will be presented in the above form. The operator ideal2list allows one to convert an ideal basis into a conventional REDUCE list.

##### Operators

Because of syntactical restrictions in REDUCE, special operators have to be used for ideal arithmetic:

.+ ideal sum (infix)
.* ideal product (infix)
.: ideal quotient (infix)
./ ideal quotient (infix)
.= ideal equality test (infix)
subset ideal inclusion test (infix)
intersection ideal intersection (prefix,binary)
member test for membership in an ideal
(infix: polynomial and ideal)
gb Groebner basis of an ideal (prefix, unary)
ideal2list convert ideal basis to polynomial list
(prefix,unary)

Example:

I(x+y,x^2) .* I(x-z);
2 2 2
I(X + X*Y - X*Z - Y*Z,X*Y - Y *Z)

The test operators return the values 1 (=true) or 0 (=false) such that they can be used in REDUCE if − then − else statements directly.

The results of sum,product,quotient,intersction are ideals represented by their Gröbner basis in the current setting and term order. The term order can be modiﬁed using the operator torder from the Gröbner package. Note that ideal equality cannot be tested with the REDUCE equal sign:

I(x,y) = I(y,x) is false
I(x,y) .= I(y,x) is true

#### 16.31.4 Algorithms

The operators groebner, preduce and idealquotient of the REDUCE Gröbner package support the basic algorithms:

GB(Iu1,u2...) → groebner({u1,u2...},{x,...})

p ∈ I1 → p = 0 mod I1

I1 : I(p) → (I1 ⋂ I(p))∕p elementwise

On top of these the Ideals package implements the following operations:

I(u1,u2...) + I(v1,v2...) → GB(I(u1,u2...,v1,v2...))

I(u1,u2...) ∗ I(v1,v2...) → GB(I(u1 ∗ v1,u1 ∗ v2,...,u2 ∗ v1,u2 ∗ v2...))

I1 ⋂ I2 → Q[x,...]⋂ GBlex(t ∗ I1 + (1 − t) ∗ I2,{t,x,..})

I1 : I(p1,p2,...) → I1 : I(p1)⋂ I1 : I(p2)⋂ ...

I1 = I2 → GB(I1) = GB(I2)

I1 ⊆ I2 → ui ∈ I2 ∀ ui ∈ I1 = I(u1,u2...)

#### 16.31.5 Examples

Please consult the ﬁle ideals.tst.

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