Generic Multivariate Polynomials

AUTHORS:

• David Joyner: first version
• William Stein: use dict’s instead of lists
• Martin Albrecht malb@informatik.uni-bremen.de: some functions added
• William Stein (2006-02-11): added better __div__ behavior.
• Kiran S. Kedlaya (2006-02-12): added Macaulay2 analogues of some Singular features
• William Stein (2006-04-19): added e.g., f[1,3] to get coeff of \(xy^3\); added examples of the new R.x,y = PolynomialRing(QQ,2) notation.
• Martin Albrecht: improved singular coercions (restructured class hierarchy) and added ETuples
• Robert Bradshaw (2007-08-14): added support for coercion of polynomials in a subset of variables (including multi-level univariate rings)
• Joel B. Mohler (2008-03): Refactored interactions with ETuples.

EXAMPLES:

We verify Lagrange’s four squares identity:

sage: R.<a0,a1,a2,a3,b0,b1,b2,b3> = QQbar[]
sage: (a0^2 + a1^2 + a2^2 + a3^2)*(b0^2 + b1^2 + b2^2 + b3^2) == (a0*b0 - a1*b1 - a2*b2 - a3*b3)^2 + (a0*b1 + a1*b0 + a2*b3 - a3*b2)^2 + (a0*b2 - a1*b3 + a2*b0 + a3*b1)^2 + (a0*b3 + a1*b2 - a2*b1 + a3*b0)^2
True

class sage.rings.polynomial.multi_polynomial_element.MPolynomial_element(parent, x)

Bases: sage.rings.polynomial.multi_polynomial.MPolynomial

EXAMPLES:

sage: K.<cuberoot2> = NumberField(x^3 - 2)
sage: L.<cuberoot3> = K.extension(x^3 - 3)
sage: S.<sqrt2> = L.extension(x^2 - 2)
sage: S
Number Field in sqrt2 with defining polynomial x^2 - 2 over its base field
sage: P.<x,y,z> = PolynomialRing(S) # indirect doctest

change_ring(R)

Change the base ring of this polynomial to R.

INPUT:

• R – ring or morphism.

OUTPUT: a new polynomial converted to R.

EXAMPLES:

sage: R.<x,y> = QQ[]
sage: f = x^2 + 5*y
sage: f.change_ring(GF(5))
x^2

sage: K.<w> = CyclotomicField(5)
sage: R.<x,y> = K[]
sage: f = x^2 + w*y
sage: f.change_ring(K.embeddings(QQbar)[1])
x^2 + (-0.8090169943749474? + 0.5877852522924731?*I)*y

element()

hamming_weight()

Return the number of non-zero coefficients of this polynomial.

This is also called weight, hamming_weight() or sparsity.

EXAMPLES:

sage: R.<x, y> = CC[]
sage: f = x^3 - y
sage: f.number_of_terms()
2
sage: R(0).number_of_terms()
0
sage: f = (x+y)^100
sage: f.number_of_terms()
101

The method hamming_weight() is an alias:

sage: f.hamming_weight()
101

number_of_terms()

Return the number of non-zero coefficients of this polynomial.

This is also called weight, hamming_weight() or sparsity.

EXAMPLES:

sage: R.<x, y> = CC[]
sage: f = x^3 - y
sage: f.number_of_terms()
2
sage: R(0).number_of_terms()
0
sage: f = (x+y)^100
sage: f.number_of_terms()
101

The method hamming_weight() is an alias:

sage: f.hamming_weight()
101

class sage.rings.polynomial.multi_polynomial_element.MPolynomial_polydict(parent, x)

Bases: sage.rings.polynomial.polynomial_singular_interface.Polynomial_singular_repr, sage.rings.polynomial.multi_polynomial_element.MPolynomial_element

Multivariate polynomials implemented in pure python using polydicts.

coefficient(degrees)

Return the coefficient of the variables with the degrees specified in the python dictionary degrees. Mathematically, this is the coefficient in the base ring adjoined by the variables of this ring not listed in degrees. However, the result has the same parent as this polynomial.

This function contrasts with the function monomial_coefficient which returns the coefficient in the base ring of a monomial.

INPUT:

• degrees - Can be any of:
  • a dictionary of degree restrictions
  • a list of degree restrictions (with None in the unrestricted variables)
  • a monomial (very fast, but not as flexible)

OUTPUT: element of the parent of self

See also

For coefficients of specific monomials, look at monomial_coefficient().

EXAMPLES:

sage: R.<x, y> = QQbar[]
sage: f = 2 * x * y
sage: c = f.coefficient({x:1,y:1}); c
2
sage: c.parent()
Multivariate Polynomial Ring in x, y over Algebraic Field
sage: c in PolynomialRing(QQbar, 2, names = ['x','y'])
True
sage: f = y^2 - x^9 - 7*x + 5*x*y
sage: f.coefficient({y:1})
5*x
sage: f.coefficient({y:0})
-x^9 + (-7)*x
sage: f.coefficient({x:0,y:0})
0
sage: f=(1+y+y^2)*(1+x+x^2)
sage: f.coefficient({x:0})
y^2 + y + 1
sage: f.coefficient([0,None])
y^2 + y + 1
sage: f.coefficient(x)
y^2 + y + 1
sage: # Be aware that this may not be what you think!
sage: # The physical appearance of the variable x is deceiving -- particularly if the exponent would be a variable.
sage: f.coefficient(x^0) # outputs the full polynomial
x^2*y^2 + x^2*y + x*y^2 + x^2 + x*y + y^2 + x + y + 1

sage: R.<x,y> = RR[]
sage: f=x*y+5
sage: c=f.coefficient({x:0,y:0}); c
5.00000000000000
sage: parent(c)
Multivariate Polynomial Ring in x, y over Real Field with 53 bits of precision

AUTHORS:

• Joel B. Mohler (2007-10-31)

constant_coefficient()

Return the constant coefficient of this multivariate polynomial.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.constant_coefficient()
5
sage: f = 3*x^2
sage: f.constant_coefficient()
0

degree(x=None, std_grading=False)

Return the degree of self in x, where x must be one of the generators for the parent of self.

INPUT:

• x - multivariate polynomial (a generator of the parent

  of self). If x is not specified (or is None), return the total degree, which is the maximum degree of any monomial. Note that a weighted term ordering alters the grading of the generators of the ring; see the tests below. To avoid this behavior, set the optional argument std_grading=True.

OUTPUT: integer

EXAMPLES:

sage: R.<x,y> = RR[]
sage: f = y^2 - x^9 - x
sage: f.degree(x)
9
sage: f.degree(y)
2
sage: (y^10*x - 7*x^2*y^5 + 5*x^3).degree(x)
3
sage: (y^10*x - 7*x^2*y^5 + 5*x^3).degree(y)
10

Note that total degree takes into account if we are working in a polynomial ring with a weighted term order.

sage: R = PolynomialRing(QQ,'x,y',order=TermOrder('wdeglex',(2,3)))
sage: x,y = R.gens()
sage: x.degree()
2
sage: y.degree()
3
sage: x.degree(y),x.degree(x),y.degree(x),y.degree(y)
(0, 1, 0, 1)
sage: f = (x^2*y+x*y^2)
sage: f.degree(x)
2
sage: f.degree(y)
2
sage: f.degree()
8
sage: f.degree(std_grading=True)
3

Note that if x is not a generator of the parent of self, for example if it is a generator of a polynomial algebra which maps naturally to this one, then it is converted to an element of this algebra. (This fixes the problem reported in trac ticket #17366.)

sage: x, y = ZZ['x','y'].gens()
sage: GF(3037000453)['x','y'].gen(0).degree(x)
1

sage: x0, y0 = QQ['x','y'].gens()
sage: GF(3037000453)['x','y'].gen(0).degree(x0)
Traceback (most recent call last):
...
TypeError: x must canonically coerce to parent

sage: GF(3037000453)['x','y'].gen(0).degree(x^2)
Traceback (most recent call last):
...
TypeError: x must be one of the generators of the parent

degrees()

Returns a tuple (precisely - an ETuple) with the degree of each variable in this polynomial. The list of degrees is, of course, ordered by the order of the generators.

EXAMPLES:

sage: R.<x,y,z>=PolynomialRing(QQbar)
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.degrees()
(2, 2, 0)
sage: f = x^2+z^2
sage: f.degrees()
(2, 0, 2)
sage: f.total_degree()  # this simply illustrates that total degree is not the sum of the degrees
2
sage: R.<x,y,z,u>=PolynomialRing(QQbar)
sage: f=(1-x)*(1+y+z+x^3)^5
sage: f.degrees()
(16, 5, 5, 0)
sage: R(0).degrees()
(0, 0, 0, 0)

dict()

Return underlying dictionary with keys the exponents and values the coefficients of this polynomial.

exponents(as_ETuples=True)

Return the exponents of the monomials appearing in self.

INPUT:

• as_ETuples (default: True): return the list of exponents as a list of ETuples.

OUTPUT:

Return the list of exponents as a list of ETuples or tuples.

EXAMPLES:

sage: R.<a,b,c> = PolynomialRing(QQbar, 3)
sage: f = a^3 + b + 2*b^2
sage: f.exponents()
[(3, 0, 0), (0, 2, 0), (0, 1, 0)]

Be default the list of exponents is a list of ETuples:

sage: type(f.exponents()[0])
<type 'sage.rings.polynomial.polydict.ETuple'>
sage: type(f.exponents(as_ETuples=False)[0])
<... 'tuple'>

factor(proof=None)

Compute the irreducible factorization of this polynomial.

INPUT:

• proof'' - insist on provably correct results (default: ``True unless explicitly disabled for the "polynomial" subsystem with sage.structure.proof.proof.WithProof.)

integral(var=None)

Integrates self with respect to variable var.

Note

The integral is always chosen so the constant term is 0.

If var is not one of the generators of this ring, integral(var) is called recursively on each coefficient of this polynomial.

EXAMPLES:

On polynomials with rational coefficients:

sage: x, y = PolynomialRing(QQ, 'x, y').gens()
sage: ex = x*y + x - y
sage: it = ex.integral(x); it
1/2*x^2*y + 1/2*x^2 - x*y
sage: it.parent() == x.parent()
True

On polynomials with coefficients in power series:

sage: R.<t> = PowerSeriesRing(QQbar)
sage: S.<x, y> = PolynomialRing(R)
sage: f = (t^2 + O(t^3))*x^2*y^3 + (37*t^4 + O(t^5))*x^3
sage: f.parent()
Multivariate Polynomial Ring in x, y over Power Series Ring in t over Algebraic Field
sage: f.integral(x)   # with respect to x
(1/3*t^2 + O(t^3))*x^3*y^3 + (37/4*t^4 + O(t^5))*x^4
sage: f.integral(x).parent()
Multivariate Polynomial Ring in x, y over Power Series Ring in t over Algebraic Field

sage: f.integral(y)   # with respect to y
(1/4*t^2 + O(t^3))*x^2*y^4 + (37*t^4 + O(t^5))*x^3*y
sage: f.integral(t)   # with respect to t (recurses into base ring)
(1/3*t^3 + O(t^4))*x^2*y^3 + (37/5*t^5 + O(t^6))*x^3

inverse_of_unit()

Return the inverse of a unit in a ring.

is_constant()

True if polynomial is constant, and False otherwise.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.is_constant()
False
sage: g = 10*x^0
sage: g.is_constant()
True

is_generator()

Returns True if self is a generator of it’s parent.

EXAMPLES:

sage: R.<x,y>=QQbar[]
sage: x.is_generator()
True
sage: (x+y-y).is_generator()
True
sage: (x*y).is_generator()
False

is_homogeneous()

Return True if self is a homogeneous polynomial.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: (x+y).is_homogeneous()
True
sage: (x.parent()(0)).is_homogeneous()
True
sage: (x+y^2).is_homogeneous()
False
sage: (x^2 + y^2).is_homogeneous()
True
sage: (x^2 + y^2*x).is_homogeneous()
False
sage: (x^2*y + y^2*x).is_homogeneous()
True

is_monomial()

Returns True if self is a monomial, which we define to be a product of generators with coefficient 1.

Use is_term to allow the coefficient to not be 1.

EXAMPLES:

sage: R.<x,y>=QQbar[]
sage: x.is_monomial()
True
sage: (x+2*y).is_monomial()
False
sage: (2*x).is_monomial()
False
sage: (x*y).is_monomial()
True

To allow a non-1 leading coefficient, use is_term():

sage: (2*x*y).is_term()
True
sage: (2*x*y).is_monomial()
False

is_term()

Returns True if self is a term, which we define to be a product of generators times some coefficient, which need not be 1.

Use is_monomial() to require that the coefficient be 1.

EXAMPLES:

sage: R.<x,y>=QQbar[]
sage: x.is_term()
True
sage: (x+2*y).is_term()
False
sage: (2*x).is_term()
True
sage: (7*x^5*y).is_term()
True

To require leading coefficient 1, use is_monomial():

sage: (2*x*y).is_monomial()
False
sage: (2*x*y).is_term()
True

is_univariate()

Returns True if this multivariate polynomial is univariate and False otherwise.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.is_univariate()
False
sage: g = f.subs({x:10}); g
700*y^2 + (-2)*y + 305
sage: g.is_univariate()
True
sage: f = x^0
sage: f.is_univariate()
True

lc()

Returns the leading coefficient of self i.e., self.coefficient(self.lm())

EXAMPLES:

sage: R.<x,y,z>=QQbar[]
sage: f=3*x^2-y^2-x*y
sage: f.lc()
3

lift(I)

given an ideal I = (f_1,…,f_r) and some g (== self) in I, find s_1,…,s_r such that g = s_1 f_1 + … + s_r f_r

ALGORITHM: Use Singular.

EXAMPLES:

sage: A.<x,y> = PolynomialRing(CC,2,order='degrevlex')
sage: I = A.ideal([x^10 + x^9*y^2, y^8 - x^2*y^7 ])
sage: f = x*y^13 + y^12
sage: M = f.lift(I)
sage: M
[y^7, x^7*y^2 + x^8 + x^5*y^3 + x^6*y + x^3*y^4 + x^4*y^2 + x*y^5 + x^2*y^3 + y^4]
sage: sum( map( mul , zip( M, I.gens() ) ) ) == f
True

lm()

Returns the lead monomial of self with respect to the term order of self.parent().

EXAMPLES:

sage: R.<x,y,z>=PolynomialRing(GF(7),3,order='lex')
sage: (x^1*y^2 + y^3*z^4).lm()
x*y^2
sage: (x^3*y^2*z^4 + x^3*y^2*z^1).lm()
x^3*y^2*z^4

sage: R.<x,y,z>=PolynomialRing(CC,3,order='deglex')
sage: (x^1*y^2*z^3 + x^3*y^2*z^0).lm()
x*y^2*z^3
sage: (x^1*y^2*z^4 + x^1*y^1*z^5).lm()
x*y^2*z^4

sage: R.<x,y,z>=PolynomialRing(QQbar,3,order='degrevlex')
sage: (x^1*y^5*z^2 + x^4*y^1*z^3).lm()
x*y^5*z^2
sage: (x^4*y^7*z^1 + x^4*y^2*z^3).lm()
x^4*y^7*z

lt()

Returns the leading term of self i.e., self.lc()*self.lm(). The notion of “leading term” depends on the ordering defined in the parent ring.

EXAMPLES:

sage: R.<x,y,z>=PolynomialRing(QQbar)
sage: f=3*x^2-y^2-x*y
sage: f.lt()
3*x^2
sage: R.<x,y,z>=PolynomialRing(QQbar,order="invlex")
sage: f=3*x^2-y^2-x*y
sage: f.lt()
-y^2

monomial_coefficient(mon)

Return the coefficient in the base ring of the monomial mon in self, where mon must have the same parent as self.

This function contrasts with the function coefficient which returns the coefficient of a monomial viewing this polynomial in a polynomial ring over a base ring having fewer variables.

INPUT:

• mon - a monomial

OUTPUT: coefficient in base ring

See also

For coefficients in a base ring of fewer variables, look at coefficient().

EXAMPLES:

The parent of the return is a member of the base ring.

sage: R.<x,y>=QQbar[]

The parent of the return is a member of the base ring.

sage: f = 2 * x * y
sage: c = f.monomial_coefficient(x*y); c
2
sage: c.parent()
Algebraic Field

sage: f = y^2 + y^2*x - x^9 - 7*x + 5*x*y
sage: f.monomial_coefficient(y^2)
1
sage: f.monomial_coefficient(x*y)
5
sage: f.monomial_coefficient(x^9)
-1
sage: f.monomial_coefficient(x^10)
0

sage: var('a')
a
sage: K.<a> = NumberField(a^2+a+1)
sage: P.<x,y> = K[]
sage: f=(a*x-1)*((a+1)*y-1); f
-x*y + (-a)*x + (-a - 1)*y + 1
sage: f.monomial_coefficient(x)
-a

monomials()

Returns the list of monomials in self. The returned list is decreasingly ordered by the term ordering of self.parent().

OUTPUT: list of MPolynomials representing Monomials

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.monomials()
[x^2*y^2, x^2, y, 1]

sage: R.<fx,fy,gx,gy> = QQbar[]
sage: F = ((fx*gy - fy*gx)^3)
sage: F
-fy^3*gx^3 + 3*fx*fy^2*gx^2*gy + (-3)*fx^2*fy*gx*gy^2 + fx^3*gy^3
sage: F.monomials()
[fy^3*gx^3, fx*fy^2*gx^2*gy, fx^2*fy*gx*gy^2, fx^3*gy^3]
sage: F.coefficients()
[-1, 3, -3, 1]
sage: sum(map(mul,zip(F.coefficients(),F.monomials()))) == F
True

nvariables()

Number of variables in this polynomial

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.nvariables ()
2
sage: g = f.subs({x:10}); g
700*y^2 + (-2)*y + 305
sage: g.nvariables ()
1

quo_rem(right)

Returns quotient and remainder of self and right.

EXAMPLES:

sage: R.<x,y> = CC[]
sage: f = y*x^2 + x + 1
sage: f.quo_rem(x)
(x*y + 1.00000000000000, 1.00000000000000)

sage: R = QQ['a','b']['x','y','z']
sage: p1 = R('a + (1+2*b)*x*y + (3-a^2)*z')
sage: p2 = R('x-1')
sage: p1.quo_rem(p2)
((2*b + 1)*y, (2*b + 1)*y + (-a^2 + 3)*z + a)

sage: R.<x,y> = Qp(5)[]
sage: x.quo_rem(y)
Traceback (most recent call last):
...
TypeError: no conversion of this ring to a Singular ring defined

ALGORITHM: Use Singular.

reduce(I)

Reduce this polynomial by the polynomials in \(I\).

INPUT:

• I - a list of polynomials or an ideal

EXAMPLES:

sage: P.<x,y,z> = QQbar[]
sage: f1 = -2 * x^2 + x^3
sage: f2 = -2 * y + x* y
sage: f3 = -x^2 + y^2
sage: F = Ideal([f1,f2,f3])
sage: g = x*y - 3*x*y^2
sage: g.reduce(F)
(-6)*y^2 + 2*y
sage: g.reduce(F.gens())
(-6)*y^2 + 2*y

sage: f = 3*x
sage: f.reduce([2*x,y])
0

sage: k.<w> = CyclotomicField(3)
sage: A.<y9,y12,y13,y15> = PolynomialRing(k)
sage: J = [ y9 + y12]
sage: f = y9 - y12; f.reduce(J)
-2*y12
sage: f = y13*y15; f.reduce(J)
y13*y15
sage: f = y13*y15 + y9 - y12; f.reduce(J)
y13*y15 - 2*y12

Make sure the remainder returns the correct type, fixing trac ticket #13903:

sage: R.<y1,y2>=PolynomialRing(Qp(5),2, order='lex')
sage: G=[y1^2 + y2^2, y1*y2 + y2^2, y2^3]
sage: type((y2^3).reduce(G))
<class 'sage.rings.polynomial.multi_polynomial_element.MPolynomial_polydict'>

resultant(other, variable=None)

Compute the resultant of self and other with respect to variable.

If a second argument is not provided, the first variable of self.parent() is chosen.

For inexact rings or rings not available in Singular, this computes the determinant of the Sylvester matrix.

INPUT:

• other – polynomial in self.parent()
• variable – (optional) variable (of type polynomial) in self.parent()

EXAMPLES:

sage: P.<x,y> = PolynomialRing(QQ, 2)
sage: a = x + y
sage: b = x^3 - y^3
sage: a.resultant(b)
-2*y^3
sage: a.resultant(b, y)
2*x^3

subs(fixed=None, **kw)

Fixes some given variables in a given multivariate polynomial and returns the changed multivariate polynomials. The polynomial itself is not affected. The variable,value pairs for fixing are to be provided as a dictionary of the form {variable:value}.

This is a special case of evaluating the polynomial with some of the variables constants and the others the original variables.

INPUT:

• fixed - (optional) dictionary of inputs
• **kw - named parameters

OUTPUT: new MPolynomial

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = x^2 + y + x^2*y^2 + 5
sage: f((5,y))
25*y^2 + y + 30
sage: f.subs({x:5})
25*y^2 + y + 30

total_degree()

Return the total degree of self, which is the maximum degree of any monomial in self.

EXAMPLES:

sage: R.<x,y,z> = QQbar[]
sage: f=2*x*y^3*z^2
sage: f.total_degree()
6
sage: f=4*x^2*y^2*z^3
sage: f.total_degree()
7
sage: f=99*x^6*y^3*z^9
sage: f.total_degree()
18
sage: f=x*y^3*z^6+3*x^2
sage: f.total_degree()
10
sage: f=z^3+8*x^4*y^5*z
sage: f.total_degree()
10
sage: f=z^9+10*x^4+y^8*x^2
sage: f.total_degree()
10

univariate_polynomial(R=None)

Returns a univariate polynomial associated to this multivariate polynomial.

INPUT:

• R - (default: None) PolynomialRing

If this polynomial is not in at most one variable, then a ValueError exception is raised. This is checked using the is_univariate() method. The new Polynomial is over the same base ring as the given MPolynomial.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.univariate_polynomial()
Traceback (most recent call last):
...
TypeError: polynomial must involve at most one variable
sage: g = f.subs({x:10}); g
700*y^2 + (-2)*y + 305
sage: g.univariate_polynomial ()
700*y^2 - 2*y + 305
sage: g.univariate_polynomial(PolynomialRing(QQ,'z'))
700*z^2 - 2*z + 305

variable(i)

Returns \(i\)-th variable occurring in this polynomial.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.variable(0)
x
sage: f.variable(1)
y

variables()

Returns the tuple of variables occurring in this polynomial.

EXAMPLES:

sage: R.<x,y> = QQbar[]
sage: f = 3*x^2 - 2*y + 7*x^2*y^2 + 5
sage: f.variables()
(x, y)
sage: g = f.subs({x:10}); g
700*y^2 + (-2)*y + 305
sage: g.variables()
(y,)

sage.rings.polynomial.multi_polynomial_element.degree_lowest_rational_function(r, x)

Return the difference of valuations of r with respect to variable x.

INPUT:

• r – a multivariate rational function
• x – a multivariate polynomial ring generator x

OUTPUT:

• integer – the difference val_x(p) - val_x(q) where r = p/q

Note

This function should be made a method of the FractionFieldElement class.

EXAMPLES:

sage: R1 = PolynomialRing(FiniteField(5), 3, names = ["a","b","c"])
sage: F = FractionField(R1)
sage: a,b,c = R1.gens()
sage: f = 3*a*b^2*c^3+4*a*b*c
sage: g = a^2*b*c^2+2*a^2*b^4*c^7

Consider the quotient \(f/g = \frac{4 + 3 bc^{2}}{ac + 2 ab^{3}c^{6}}\) (note the cancellation).

sage: r = f/g; r
(-b*c^2 + 2)/(a*b^3*c^6 - 2*a*c)
sage: degree_lowest_rational_function(r,a)
-1
sage: degree_lowest_rational_function(r,b)
0
sage: degree_lowest_rational_function(r,c)
-1

sage.rings.polynomial.multi_polynomial_element.is_MPolynomial(x)