def eliminate(polys, elim_variables, **kwds):
r"""
Compute a Groebner basis with respect to an elimination order defined by
the given variables.
INPUT:
- ``polys`` -- an ideal or a polynomial sequence.
- ``elim_variables`` -- the variables to eliminate.
- ``force_elim`` -- integer (default: `1`).
- ``kwds`` -- same as in :func:`groebner_basis`.
OUTPUT: a Groebner basis of the elimination ideal.
EXAMPLES::
sage: R.<x,y,t,s,z> = PolynomialRing(QQ,5)
sage: I = R * [x-t,y-t^2,z-t^3,s-x+y^3]
sage: import fgb_sage # optional - fgb_sage
sage: gb = fgb_sage.eliminate(I, [t,s], verbosity=0) # optional - fgb_sage, random
open simulation
sage: gb # optional - fgb_sage
[x^2 - y, x*y - z, y^2 - x*z]
sage: gb.is_groebner() # optional - fgb_sage
True
sage: gb.ideal() == I.elimination_ideal([t,s]) # optional - fgb_sage
True
.. NOTE::
In some cases, this function fails to set the correct elimination
order, see :trac:`24981`. This was fixed in Sage 8.7.
"""
kwds.setdefault('force_elim', 1)
polyseq = PolynomialSequence(polys)
ring = polyseq.ring()
elim_variables = set(elim_variables)
block1 = [x for x in ring.gens() if x in elim_variables]
block2 = [x for x in ring.gens() if x not in elim_variables]
from sage.rings.polynomial.term_order import TermOrder
if len(block1) == 0 or len(block2) == 0:
t = TermOrder("degrevlex", ring.ngens())
else:
t = TermOrder("degrevlex", len(block1)) + TermOrder(
"degrevlex", len(block2))
if t == ring.term_order() and set(
ring.gens()[:len(block1)]) == elim_variables:
return groebner_basis(polyseq, **kwds)
else:
block_ring = ring.change_ring(names=block1 + block2, order=t)
gb = groebner_basis(PolynomialSequence(block_ring, polyseq), **kwds)
return PolynomialSequence(ring, gb, immutable=True)
def groebner_basis(polys, **kwds):
r"""
Compute a Groebner basis of an ideal using FGb.
Supported term orders of the underlying polynomial ring are ``degrevlex``
orders, as well as block orders with two ``degrevlex`` blocks (elimination
orders). Supported coefficient fields are QQ and finite prime fields of
size up to ``MAX_PRIME`` `= 65521 < 2^16`.
INPUT:
- ``polys`` -- an ideal or a polynomial sequence, the generators of an
ideal.
- ``threads`` -- integer (default: `1`); only seems to work in positive
characteristic.
- ``force_elim`` -- integer (default: `0`); if ``force_elim=1``, then the
computation will return only the result of the elimination, if an
elimination order is used.
- ``verbosity`` -- integer (default: `1`), display progress info.
- ``matrix_bound`` -- integer (default: `500000`); this is is the maximal
size of the matrices generated by F4. This value can be increased
according to available memory.
- ``max_base`` -- integer (default: `100000`); maximum number of
polynomials in output.
OUTPUT: the Groebner basis.
EXAMPLES:
This example computes a Groebner basis with respect to an elimination
order::
sage: R = PolynomialRing(QQ, 5, 'x', order="degrevlex(2),degrevlex(3)")
sage: I = sage.rings.ideal.Cyclic(R)
sage: import fgb_sage # optional fgb_sage
sage: gb = fgb_sage.groebner_basis(I) # optional fgb_sage, random
...
sage: gb.is_groebner(), gb.ideal() == I # optional fgb_sage
(True, True)
Over finite fields, parallel computations are supported::
sage: R = PolynomialRing(GF(fgb_sage.MAX_PRIME), 4, 'x') # optional fgb_sage
sage: I = sage.rings.ideal.Katsura(R) # optional fgb_sage
sage: gb = fgb_sage.groebner_basis(I, threads=2, verbosity=0) # optional fgb_sage, random
sage: gb.is_groebner(), gb.ideal() == I # optional fgb_sage
(True, True)
If :func:`fgb_sage.groebner_basis` is called with an ideal, the result is
cached on :meth:`MPolynomialIdeal.groebner_basis` so that other
computations on the ideal do not need to recompute a Groebner basis::
sage: I.groebner_basis.is_in_cache() # optional fgb_sage
True
sage: I.groebner_basis() is gb # optional fgb_sage
True
However, note that ``gb.ideal()`` returns a new ideal and, thus, does not
have a Groebner basis in cache::
sage: gb.ideal().groebner_basis.is_in_cache() # optional fgb_sage
False
TESTS:
Check that the result is not cached if it is only a basis of an
elimination ideal::
sage: R = PolynomialRing(QQ, 5, 'x', order="degrevlex(2),degrevlex(3)")
sage: I = sage.rings.ideal.Cyclic(R)
sage: import fgb_sage
sage: gb = fgb_sage.groebner_basis(I, force_elim=1) # optional fgb_sage, random
...
sage: I.groebner_basis.is_in_cache() # optional fgb_sage
False
"""
kwds.setdefault('force_elim', 0)
kwds.setdefault('threads', 1)
kwds.setdefault('matrix_bound', 500000)
kwds.setdefault('verbosity', 1)
kwds.setdefault('max_base', 100000)
polyseq = PolynomialSequence(polys)
ring = polyseq.ring()
field = ring.base_ring()
if not field.is_prime_field():
raise NotImplementedError("base ring must be QQ or finite prime field")
if field.characteristic() > MAX_PRIME:
raise NotImplementedError("maximum prime field size is %s" % MAX_PRIME)
blocks = ring.term_order().blocks()
if not (len(blocks) <= 2 and all(order.name() == 'degrevlex'
for order in blocks)):
raise NotImplementedError(
"term order must be Degree-Reverse-Lexicographic block order with at most 2 blocks"
)
n_elim_variables = len(blocks[0])
gb = None
if field.characteristic() == 0:
from ._fgb_sage_int import fgb_eliminate
gb = fgb_eliminate(polyseq, n_elim_variables, **kwds)
else:
from ._fgb_sage_modp import fgb_eliminate
gb = fgb_eliminate(polyseq, n_elim_variables, **kwds)
from sage.rings.polynomial.multi_polynomial_ideal import MPolynomialIdeal
if isinstance(polys, MPolynomialIdeal) and not kwds['force_elim']:
if not polys.groebner_basis.is_in_cache():
polys.groebner_basis.set_cache(gb)
return gb
