Domains of multivariate polynomials
Dom::MultivariatePolynomial(<Vars, <R, <Order>>
>)
Dom::MultivariatePolynomial(Vars, R, Order)(p
)
Dom::MultivariatePolynomial(Vars, R, Order)(lm
)
Dom::MultivariatePolynomial(Vars, R, ..)
creates
the domain of multivariate polynomials in the variable list Vars
over
the commutative ring R
in distributed representation.
Dom::MultivariatePolynomial
represents multivariate
polynomials over arbitrary commutative rings.
All usual algebraic and arithmetical polynomial operations are implemented, including Gröbner basis computation and some classical construction tools used in invariant theory.
Note: It is highly recommend to use only coefficient rings with unique zero representation. Otherwise it may happen that, e.g., a polynomial division will not terminate or a wrong degree will be returned. 
Dom::MultivariatePolynomial(Vars, R, Order)
creates
a domain of multivariate polynomials in the variable list Vars
over
a domain R
of category Cat::CommutativeRing
in sparse distributed
representation with respect to the monomial ordering Order
.
Dom::MultivariatePolynomial()
creates a polynomial
domain in the variable list [x,y,z]
over the domain Dom::ExpressionField
(normal)
with
respect to the lexicographic monomial ordering.
Dom::MultivariatePolynomial(Vars)
generates
the polynomial domain in the variable list Vars
over
the domain Dom::ExpressionField(normal)
with respect
to the lexicographic monomial ordering is created.
Note:
Only commutative coefficient rings of type 
In contrast to the domain Dom::DistributedPolynomial
, Dom::MultivariatePolynomial
accepts
only identifiers (DOM_IDENT
) as indeterminates. This restriction
enables some further methods described below.
Please note: For reasons of efficiency not all methods check their arguments, not even at the interactive level. In particular this is true for many access methods, converting methods and technical methods. This may cause strange error messages.
If Vars
has a single variable, then Cat::UnivariatePolynomial
(R)
,
else Cat::Polynomial
(R)
.
To create the ring of multivariate polynomials in x
, y
and z
over
the rationals one may define
MP := Dom::MultivariatePolynomial([x, y, z], Dom::Rational)
The elementary symmetric polynomials of this domain are
s1 := MP(x + y + z)
s2 := MP(x*y + x*z + y*z)
s3:=MP(x*y*z)
A polynomial is called symmetric if it remains unchanged under every possible permutation of variables as, e.g.:
s3=s3(MP(y), MP(z), MP(x))
These polynomials arise naturally in studying the roots of a
polynomial. To show this, we first have to create an univariate polynomial,
e.g., in U
over MP
, and generate
a polynomial in U
with roots in x
, y
and z
.
UP:=Dom::UnivariatePolynomial(U, MP)
f := UP((U  x)*(U  y)*(U  z))
UP(U^3)s1*UP(U^2)+s2*UP(U)+(1)^3*s3
This exemplifies that the coefficients of f
are
(elementary) symmetric polynomials in its roots.
From the fundamental theorem of symmetric polynomials we know
that every symmetric polynomial can be written uniquely as a polynomial
in the elementary symmetric polynomials. Thus we can rewrite the following
symmetric polynomial s
in the elementary symmetric
polynomials s1
, s2
and s3
,
s:=MP(x^3*y+x^3*z+x*y^3+x*z^3+y^3*z+y*z^3)
S:=MP::rewritePoly(s,[s1=S1,s2=S2,s3=S3])
where these polynomials are represented by the three new variables S1
, S2
and S3
respectively.
To see that this new polynomial S
in the new variables
indeed represents the old original polynomial s
,
we simply have to plug in the three elementary symmetric polynomials
into S
:
poly(S, Expr)(s1,s2,s3)
When one has a given list of polynomials, e.g., like:
l:=[3*s1,2*s1,s1,s3]
and one wants to sort them in an appropriate order, one may use one of the following two methods.
MP::sortList(l,Dom::MonomOrdering(DegLex(3)))
MP::stableSort(l,Dom::MonomOrdering(DegLex(3)))
In the first sorted list the order of the three polynomials of the same degree has changed, while with the second method this order remains stable.
Let be a finite (matrix) subgroup of the general linear group. Then a polynomial is called invariant underG, if for all A ∈ G
where
.
The symmetric polynomials s1
, s2
and s3
from
the previous example are invariants under the symmetric group S_{3}.
In fact, these three fundamental invariants yet generate the whole
ring of invariants of S_{3}.
Now let us examine the invariants of the famous icosahedral group. One may find a representation of this group on page 73 of H. F. Blichfeldt: Finite collineation groups, University of Chicago Press, 1917.
The group is generated from these three matrices, has 120 elements and is thus a finite subgroup, even of the special linear group SL(2, ℚ(ε)). It is also well known that
is a fundamental invariant of degree 12 of this group. To declare i_{1} in MuPAD^{®} one has first to define the polynomial domain.
MP:=Dom::MultivariatePolynomial([x1,x2],Dom::Rational)
i1:=MP(x1*x2^(11)11*x1^6*x2^6x1^(11)*x2)
From the invariant i_{1} one can compute a further fundamental invariant i_{2} with
i2:=MP::hessianDet(i1)
But to get more simple coefficients we choose i_{2} as
i2:=1/121*MP::hessianDet(i1)
instead. Similar we obtain a third fundamental invariant i_{3} with
i3:=1/20*MP::jacobianDet([i1,i2])
In contrast to the symmetric groups, where all invariants can be uniquely represented by the fundamental invariants, the fundamental invariants of this group have an algebraic relation, a socalled syzygy between them. It is possible to represent i_{3}^{2} in two ways:
MP::rewritePoly(i3^2,[i1=I1,i2=I2,i3=I3])
MP::rewritePoly(i3^2,[i1=I1,i2=I2,i3=I3],Unsorted)
And hence we get the syzygy:
MP::rewritePoly(i3^2,[i1=I1,i2=I2,i3=I3],Unsorted) MP::rewritePoly(i3^2,[i1=I1,i2=I2,i3=I3]) = 0

A list of indeterminates. Default: 

A commutative ring, i.e., a domain of category 

A monomial ordering, i.e., one of the predefined orderings 

A polynomial or a polynomial expression. 

List of monomials, which are represented as lists containing the coefficients together with the exponents or exponent vectors. 
"characteristic"  The characteristic of this domain. 
"coeffRing"  The coefficient ring of this domain as defined by the
parameter 
"key"  The name of the domain created. 
"one"  The neutral element w.r.t. 
"ordering"  The monomial ordering defined by the parameter 
"variables"  The list of variables defined by the parameter 
"zero"  The neutral element w.r.t. 
[1] Winfried Fakler. "Algorithmen zur symbolischen Lösung homogener linearer Differentialgleichungen". Diplomarbeit, Universität Karlsruhe, 1994.