Quotient fields

class sage.categories.quotient_fields.QuotientFields(s=None)

Bases: sage.categories.category_singleton.Category_singleton

The category of quotient fields over an integral domain

EXAMPLES:

sage: QuotientFields()
Category of quotient fields
sage: QuotientFields().super_categories()
[Category of fields]

TESTS:

sage: TestSuite(QuotientFields()).run()
class ElementMethods
denominator()

Constructor for abstract methods

EXAMPLES:

sage: def f(x):
...       "doc of f"
...       return 1
...
sage: x = abstract_method(f); x
<abstract method f at ...>
sage: x.__doc__
'doc of f'
sage: x.__name__
'f'
sage: x.__module__
'__main__'
derivative(*args)

The derivative of this rational function, with respect to variables supplied in args.

Multiple variables and iteration counts may be supplied; see documentation for the global derivative() function for more details.

See also

_derivative()

EXAMPLES:

sage: F.<x> = Frac(QQ['x'])
sage: (1/x).derivative()
-1/x^2
sage: (x+1/x).derivative(x, 2)
2/x^3
sage: F.<x,y> = Frac(QQ['x,y'])
sage: (1/(x+y)).derivative(x,y)
2/(x^3 + 3*x^2*y + 3*x*y^2 + y^3)
factor(*args, **kwds)

Return the factorization of self over the base ring.

INPUT:

  • *args - Arbitrary arguments suitable over the base ring
  • **kwds - Arbitrary keyword arguments suitable over the base ring

OUTPUT:

  • Factorization of self over the base ring

EXAMPLES:

sage: K.<x> = QQ[]
sage: f = (x^3+x)/(x-3)
sage: f.factor()
(x - 3)^-1 * x * (x^2 + 1)

Here is an example to show that ticket #7868 has been resolved:

sage: R.<x,y> = GF(2)[]
sage: f = x*y/(x+y)
sage: f.factor()
(x + y)^-1 * y * x
gcd(other)

Greatest common divisor

NOTE:

In a field, the greatest common divisor is not very informative, as it is only determined up to a unit. But in the fraction field of an integral domain that provides both gcd and lcm, it is possible to be a bit more specific and define the gcd uniquely up to a unit of the base ring (rather than in the fraction field).

AUTHOR:

  • Simon King (2011-02): See trac ticket #10771

EXAMPLES:

sage: R.<x>=QQ[]
sage: p = (1+x)^3*(1+2*x^2)/(1-x^5)
sage: q = (1+x)^2*(1+3*x^2)/(1-x^4)
sage: factor(p)
(-2) * (x - 1)^-1 * (x + 1)^3 * (x^2 + 1/2) * (x^4 + x^3 + x^2 + x + 1)^-1
sage: factor(q)
(-3) * (x - 1)^-1 * (x + 1) * (x^2 + 1)^-1 * (x^2 + 1/3)
sage: gcd(p,q)
(x + 1)/(x^7 + x^5 - x^2 - 1)
sage: factor(gcd(p,q))
(x - 1)^-1 * (x + 1) * (x^2 + 1)^-1 * (x^4 + x^3 + x^2 + x + 1)^-1
sage: factor(gcd(p,1+x))
(x - 1)^-1 * (x + 1) * (x^4 + x^3 + x^2 + x + 1)^-1
sage: factor(gcd(1+x,q))
(x - 1)^-1 * (x + 1) * (x^2 + 1)^-1

TESTS:

The following tests that the fraction field returns a correct gcd even if the base ring does not provide lcm and gcd:

sage: R = ZZ.extension(x^2+5,names='q'); R
Order in Number Field in q with defining polynomial x^2 + 5
sage: R.1
q
sage: gcd(R.1,R.1)
Traceback (most recent call last):
...
TypeError: unable to find gcd
sage: (R.1/1).parent()
Number Field in q with defining polynomial x^2 + 5
sage: gcd(R.1/1,R.1)
1
sage: gcd(R.1/1,0)
1
sage: gcd(R.zero(),0)
0
lcm(other)

Least common multiple

NOTE:

In a field, the least common multiple is not very informative, as it is only determined up to a unit. But in the fraction field of an integral domain that provides both gcd and lcm, it is reasonable to be a bit more specific and to define the least common multiple so that it restricts to the usual least common multiple in the base ring and is unique up to a unit of the base ring (rather than up to a unit of the fraction field).

AUTHOR:

  • Simon King (2011-02): See trac ticket #10771

EXAMPLES:

sage: R.<x>=QQ[]
sage: p = (1+x)^3*(1+2*x^2)/(1-x^5)
sage: q = (1+x)^2*(1+3*x^2)/(1-x^4)
sage: factor(p)
(-2) * (x - 1)^-1 * (x + 1)^3 * (x^2 + 1/2) * (x^4 + x^3 + x^2 + x + 1)^-1
sage: factor(q)
(-3) * (x - 1)^-1 * (x + 1) * (x^2 + 1)^-1 * (x^2 + 1/3)
sage: factor(lcm(p,q))
(x - 1)^-1 * (x + 1)^3 * (x^2 + 1/3) * (x^2 + 1/2)
sage: factor(lcm(p,1+x))
(x + 1)^3 * (x^2 + 1/2)
sage: factor(lcm(1+x,q))
(x + 1) * (x^2 + 1/3)

TESTS:

The following tests that the fraction field returns a correct lcm even if the base ring does not provide lcm and gcd:

sage: R = ZZ.extension(x^2+5,names='q'); R
Order in Number Field in q with defining polynomial x^2 + 5
sage: R.1
q
sage: lcm(R.1,R.1)
Traceback (most recent call last):
...
TypeError: unable to find lcm
sage: (R.1/1).parent()
Number Field in q with defining polynomial x^2 + 5
sage: lcm(R.1/1,R.1)
1
sage: lcm(R.1/1,0)
0
sage: lcm(R.zero(),0)
0
numerator()

Constructor for abstract methods

EXAMPLES:

sage: def f(x):
...       "doc of f"
...       return 1
...
sage: x = abstract_method(f); x
<abstract method f at ...>
sage: x.__doc__
'doc of f'
sage: x.__name__
'f'
sage: x.__module__
'__main__'
partial_fraction_decomposition(decompose_powers=True)

Decomposes fraction field element into a whole part and a list of fraction field elements over prime power denominators.

The sum will be equal to the original fraction.

INPUT:

  • decompose_powers - whether to decompose prime power

    denominators as opposed to having a single term for each irreducible factor of the denominator (default: True)

OUTPUT:

  • Partial fraction decomposition of self over the base ring.

AUTHORS:

  • Robert Bradshaw (2007-05-31)

EXAMPLES:

sage: S.<t> = QQ[]
sage: q = 1/(t+1) + 2/(t+2) + 3/(t-3); q
(6*t^2 + 4*t - 6)/(t^3 - 7*t - 6)
sage: whole, parts = q.partial_fraction_decomposition(); parts
[3/(t - 3), 1/(t + 1), 2/(t + 2)]
sage: sum(parts) == q
True
sage: q = 1/(t^3+1) + 2/(t^2+2) + 3/(t-3)^5
sage: whole, parts = q.partial_fraction_decomposition(); parts
[1/3/(t + 1), 3/(t^5 - 15*t^4 + 90*t^3 - 270*t^2 + 405*t - 243), (-1/3*t + 2/3)/(t^2 - t + 1), 2/(t^2 + 2)]
sage: sum(parts) == q
True
sage: q = 2*t / (t + 3)^2
sage: q.partial_fraction_decomposition()
(0, [2/(t + 3), -6/(t^2 + 6*t + 9)])
sage: for p in q.partial_fraction_decomposition()[1]: print p.factor()
(2) * (t + 3)^-1
(-6) * (t + 3)^-2
sage: q.partial_fraction_decomposition(decompose_powers=False)
(0, [2*t/(t^2 + 6*t + 9)])

We can decompose over a given algebraic extension:

sage: R.<x> = QQ[sqrt(2)][]
sage: r =  1/(x^4+1)
sage: r.partial_fraction_decomposition()
(0,
 [(-1/4*sqrt2*x + 1/2)/(x^2 - sqrt2*x + 1),
  (1/4*sqrt2*x + 1/2)/(x^2 + sqrt2*x + 1)])

sage: R.<x> = QQ[I][]  # of QQ[sqrt(-1)]
sage: r =  1/(x^4+1)
sage: r.partial_fraction_decomposition()
(0, [(-1/2*I)/(x^2 - I), 1/2*I/(x^2 + I)])

We can also ask Sage to find the least extension where the denominator factors in linear terms:

sage: R.<x> = QQ[]
sage: r = 1/(x^4+2)
sage: N = r.denominator().splitting_field('a')
sage: N
Number Field in a with defining polynomial x^8 - 8*x^6 + 28*x^4 + 16*x^2 + 36
sage: R1.<x1>=N[]
sage: r1 = 1/(x1^4+2)
sage: r1.partial_fraction_decomposition()
(0,
 [(-1/224*a^6 + 13/448*a^4 - 5/56*a^2 - 25/224)/(x1 - 1/28*a^6 + 13/56*a^4 - 5/7*a^2 - 25/28),
  (1/224*a^6 - 13/448*a^4 + 5/56*a^2 + 25/224)/(x1 + 1/28*a^6 - 13/56*a^4 + 5/7*a^2 + 25/28),
  (-5/1344*a^7 + 43/1344*a^5 - 85/672*a^3 - 31/672*a)/(x1 - 5/168*a^7 + 43/168*a^5 - 85/84*a^3 - 31/84*a),
  (5/1344*a^7 - 43/1344*a^5 + 85/672*a^3 + 31/672*a)/(x1 + 5/168*a^7 - 43/168*a^5 + 85/84*a^3 + 31/84*a)])

Or we may work directly over an algebraically closed field:

sage: R.<x> = QQbar[]
sage: r =  1/(x^4+1)
sage: r.partial_fraction_decomposition()
(0,
 [(-0.1767766952966369? - 0.1767766952966369?*I)/(x - 0.7071067811865475? - 0.7071067811865475?*I),
  (-0.1767766952966369? + 0.1767766952966369?*I)/(x - 0.7071067811865475? + 0.7071067811865475?*I),
  (0.1767766952966369? - 0.1767766952966369?*I)/(x + 0.7071067811865475? - 0.7071067811865475?*I),
  (0.1767766952966369? + 0.1767766952966369?*I)/(x + 0.7071067811865475? + 0.7071067811865475?*I)])

We do the best we can over inexact fields:

sage: R.<x> = RealField(20)[]
sage: q = 1/(x^2 + x + 2)^2 + 1/(x-1); q
(x^4 + 2.0000*x^3 + 5.0000*x^2 + 5.0000*x + 3.0000)/(x^5 + x^4 + 3.0000*x^3 - x^2 - 4.0000)
sage: whole, parts = q.partial_fraction_decomposition(); parts
[1.0000/(x - 1.0000), 1.0000/(x^4 + 2.0000*x^3 + 5.0000*x^2 + 4.0000*x + 4.0000)]
sage: sum(parts)
(x^4 + 2.0000*x^3 + 5.0000*x^2 + 5.0000*x + 3.0000)/(x^5 + x^4 + 3.0000*x^3 - x^2 - 4.0000)

TESTS:

We test partial fraction for irreducible denominators:

sage: R.<x> = ZZ[]
sage: q = x^2/(x-1)
sage: q.partial_fraction_decomposition()
(x + 1, [1/(x - 1)])
sage: q = x^10/(x-1)^5
sage: whole, parts = q.partial_fraction_decomposition()
sage: whole + sum(parts) == q
True

And also over finite fields (see trac #6052, #9945):

sage: R.<x> = GF(2)[]
sage: q = (x+1)/(x^3+x+1)
sage: q.partial_fraction_decomposition()
(0, [(x + 1)/(x^3 + x + 1)])

sage: R.<x> = GF(11)[]
sage: q = x + 1 + 1/(x+1) + x^2/(x^3 + 2*x + 9)
sage: q.partial_fraction_decomposition()
(x + 1, [1/(x + 1), x^2/(x^3 + 2*x + 9)])

And even the rationals:

sage: (26/15).partial_fraction_decomposition()
(1, [1/3, 2/5])
sage: (26/75).partial_fraction_decomposition()
(-1, [2/3, 3/5, 2/25])

A larger example:

sage: S.<t> = QQ[]
sage: r = t / (t^3+1)^5
sage: r.partial_fraction_decomposition()
(0,
 [-35/729/(t + 1),
  -35/729/(t^2 + 2*t + 1),
  -25/729/(t^3 + 3*t^2 + 3*t + 1),
  -4/243/(t^4 + 4*t^3 + 6*t^2 + 4*t + 1),
  -1/243/(t^5 + 5*t^4 + 10*t^3 + 10*t^2 + 5*t + 1),
  (35/729*t - 35/729)/(t^2 - t + 1),
  (25/729*t - 8/729)/(t^4 - 2*t^3 + 3*t^2 - 2*t + 1),
  (-1/81*t + 5/81)/(t^6 - 3*t^5 + 6*t^4 - 7*t^3 + 6*t^2 - 3*t + 1),
  (-2/27*t + 1/9)/(t^8 - 4*t^7 + 10*t^6 - 16*t^5 + 19*t^4 - 16*t^3 + 10*t^2 - 4*t + 1),
  (-2/27*t + 1/27)/(t^10 - 5*t^9 + 15*t^8 - 30*t^7 + 45*t^6 - 51*t^5 + 45*t^4 - 30*t^3 + 15*t^2 - 5*t + 1)])
sage: sum(r.partial_fraction_decomposition()[1]) == r
True

Some special cases:

sage: R = Frac(QQ['x']); x = R.gen()
sage: x.partial_fraction_decomposition()
(x, [])
sage: R(0).partial_fraction_decomposition()
(0, [])
sage: R(1).partial_fraction_decomposition()
(1, [])
sage: (1/x).partial_fraction_decomposition()
(0, [1/x])
sage: (1/x+1/x^3).partial_fraction_decomposition()
(0, [1/x, 1/x^3])
class QuotientFields.ParentMethods
QuotientFields.super_categories()

EXAMPLES:

sage: QuotientFields().super_categories()
[Category of fields]

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