from __future__ import print_function, division
from sympy.core import S
from sympy.polys import Poly
[docs]def dispersionset(p, q=None, *gens, **args):
r"""Compute the *dispersion set* of two polynomials.
For two polynomials `f(x)` and `g(x)` with `\deg f > 0`
and `\deg g > 0` the dispersion set `\operatorname{J}(f, g)` is defined as:
.. math::
\operatorname{J}(f, g)
& := \{a \in \mathbb{N}_0 | \gcd(f(x), g(x+a)) \neq 1\} \\
& = \{a \in \mathbb{N}_0 | \deg \gcd(f(x), g(x+a)) \geq 1\}
For a single polynomial one defines `\operatorname{J}(f) := \operatorname{J}(f, f)`.
Examples
========
>>> from sympy import poly
>>> from sympy.polys.dispersion import dispersion, dispersionset
>>> from sympy.abc import x
Dispersion set and dispersion of a simple polynomial:
>>> fp = poly((x - 3)*(x + 3), x)
>>> sorted(dispersionset(fp))
[0, 6]
>>> dispersion(fp)
6
Note that the definition of the dispersion is not symmetric:
>>> fp = poly(x**4 - 3*x**2 + 1, x)
>>> gp = fp.shift(-3)
>>> sorted(dispersionset(fp, gp))
[2, 3, 4]
>>> dispersion(fp, gp)
4
>>> sorted(dispersionset(gp, fp))
[]
>>> dispersion(gp, fp)
-oo
Computing the dispersion also works over field extensions:
>>> from sympy import sqrt
>>> fp = poly(x**2 + sqrt(5)*x - 1, x, domain='QQ<sqrt(5)>')
>>> gp = poly(x**2 + (2 + sqrt(5))*x + sqrt(5), x, domain='QQ<sqrt(5)>')
>>> sorted(dispersionset(fp, gp))
[2]
>>> sorted(dispersionset(gp, fp))
[1, 4]
We can even perform the computations for polynomials
having symbolic coefficients:
>>> from sympy.abc import a
>>> fp = poly(4*x**4 + (4*a + 8)*x**3 + (a**2 + 6*a + 4)*x**2 + (a**2 + 2*a)*x, x)
>>> sorted(dispersionset(fp))
[0, 1]
See Also
========
dispersion
References
==========
1. [ManWright94]_
2. [Koepf98]_
3. [Abramov71]_
4. [Man93]_
"""
# Check for valid input
same = False if q is not None else True
if same:
q = p
p = Poly(p, *gens, **args)
q = Poly(q, *gens, **args)
if not p.is_univariate or not q.is_univariate:
raise ValueError("Polynomials need to be univariate")
# The generator
if not p.gen == q.gen:
raise ValueError("Polynomials must have the same generator")
gen = p.gen
# We define the dispersion of constant polynomials to be zero
if p.degree() < 1 or q.degree() < 1:
return set([0])
# Factor p and q over the rationals
fp = p.factor_list()
fq = q.factor_list() if not same else fp
# Iterate over all pairs of factors
J = set([])
for s, unused in fp[1]:
for t, unused in fq[1]:
m = s.degree()
n = t.degree()
if n != m:
continue
an = s.LC()
bn = t.LC()
if not (an - bn).is_zero:
continue
# Note that the roles of `s` and `t` below are switched
# w.r.t. the original paper. This is for consistency
# with the description in the book of W. Koepf.
anm1 = s.coeff_monomial(gen**(m-1))
bnm1 = t.coeff_monomial(gen**(n-1))
alpha = (anm1 - bnm1) / S(n*bn)
if not alpha.is_integer:
continue
if alpha < 0 or alpha in J:
continue
if n > 1 and not (s - t.shift(alpha)).is_zero:
continue
J.add(alpha)
return J
[docs]def dispersion(p, q=None, *gens, **args):
r"""Compute the *dispersion* of polynomials.
For two polynomials `f(x)` and `g(x)` with `\deg f > 0`
and `\deg g > 0` the dispersion `\operatorname{dis}(f, g)` is defined as:
.. math::
\operatorname{dis}(f, g)
& := \max\{ J(f,g) \cup \{0\} \} \\
& = \max\{ \{a \in \mathbb{N} | \gcd(f(x), g(x+a)) \neq 1\} \cup \{0\} \}
and for a single polynomial `\operatorname{dis}(f) := \operatorname{dis}(f, f)`.
Note that we make the definition `\max\{\} := -\infty`.
Examples
========
>>> from sympy import poly
>>> from sympy.polys.dispersion import dispersion, dispersionset
>>> from sympy.abc import x
Dispersion set and dispersion of a simple polynomial:
>>> fp = poly((x - 3)*(x + 3), x)
>>> sorted(dispersionset(fp))
[0, 6]
>>> dispersion(fp)
6
Note that the definition of the dispersion is not symmetric:
>>> fp = poly(x**4 - 3*x**2 + 1, x)
>>> gp = fp.shift(-3)
>>> sorted(dispersionset(fp, gp))
[2, 3, 4]
>>> dispersion(fp, gp)
4
>>> sorted(dispersionset(gp, fp))
[]
>>> dispersion(gp, fp)
-oo
The maximum of an empty set is defined to be `-\infty`
as seen in this example.
Computing the dispersion also works over field extensions:
>>> from sympy import sqrt
>>> fp = poly(x**2 + sqrt(5)*x - 1, x, domain='QQ<sqrt(5)>')
>>> gp = poly(x**2 + (2 + sqrt(5))*x + sqrt(5), x, domain='QQ<sqrt(5)>')
>>> sorted(dispersionset(fp, gp))
[2]
>>> sorted(dispersionset(gp, fp))
[1, 4]
We can even perform the computations for polynomials
having symbolic coefficients:
>>> from sympy.abc import a
>>> fp = poly(4*x**4 + (4*a + 8)*x**3 + (a**2 + 6*a + 4)*x**2 + (a**2 + 2*a)*x, x)
>>> sorted(dispersionset(fp))
[0, 1]
See Also
========
dispersionset
References
==========
1. [ManWright94]_
2. [Koepf98]_
3. [Abramov71]_
4. [Man93]_
"""
J = dispersionset(p, q, *gens, **args)
if not J:
# Definition for maximum of empty set
j = S.NegativeInfinity
else:
j = max(J)
return j
