ALGORITHM:
- For now, we skip the messy minimal polynomial computation
- and instead return the dimension of the vector space spanned
- by the powers of this element. The latter is a bit more
- straightforward to compute.
+ .........
SETUP::
True
"""
- if self.is_zero() and not self.parent().is_trivial():
+ n = self.parent().dimension()
+
+ if n == 0:
+ # The minimal polynomial is an empty product, i.e. the
+ # constant polynomial "1" having degree zero.
+ return 0
+ elif self.is_zero():
# The minimal polynomial of zero in a nontrivial algebra
- # is "t"; in a trivial algebra it's "1" by convention
- # (it's an empty product).
+ # is "t", and is of degree one.
+ return 1
+ elif n == 1:
+ # If this is a nonzero element of a nontrivial algebra, it
+ # has degree at least one. It follows that, in an algebra
+ # of dimension one, the degree must be actually one.
return 1
- return self.subalgebra_generated_by().dimension()
+
+ # BEWARE: The subalgebra_generated_by() method uses the result
+ # of this method to construct a basis for the subalgebra. That
+ # means, in particular, that we cannot implement this method
+ # as ``self.subalgebra_generated_by().dimension()``.
+
+ # Algorithm: keep appending (vector representations of) powers
+ # self as rows to a matrix and echelonizing it. When its rank
+ # stops increasing, we've reached a redundancy.
+
+ # Given the special cases above, we can assume that "self" is
+ # nonzero, the algebra is nontrivial, and that its dimension
+ # is at least two.
+ M = matrix([(self.parent().one()).to_vector()])
+ old_rank = 1
+
+ # Specifying the row-reduction algorithm can e.g. help over
+ # AA because it avoids the RecursionError that gets thrown
+ # when we have to look too hard for a root.
+ #
+ # Beware: QQ supports an entirely different set of "algorithm"
+ # keywords than do AA and RR.
+ algo = None
+ from sage.rings.all import QQ
+ if self.parent().base_ring() is not QQ:
+ algo = "scaled_partial_pivoting"
+
+ for d in range(1,n):
+ M = matrix(M.rows() + [(self**d).to_vector()])
+ M.echelonize(algo)
+ new_rank = M.rank()
+ if new_rank == old_rank:
+ return new_rank
+ else:
+ old_rank = new_rank
+
+ return n
+
def left_matrix(self):
result.append( (evalue, proj(A.one()).superalgebra_element()) )
return result
- def subalgebra_generated_by(self, orthonormalize_basis=False):
+ def subalgebra_generated_by(self, **kwargs):
"""
Return the associative subalgebra of the parent EJA generated
by this element.
"""
from mjo.eja.eja_element_subalgebra import FiniteDimensionalEJAElementSubalgebra
- return FiniteDimensionalEJAElementSubalgebra(self, orthonormalize_basis)
+ return FiniteDimensionalEJAElementSubalgebra(self, **kwargs)
def subalgebra_idempotent(self):
# and continually rref() it until the rank stops going
# up. When n=10 but the dimension of the algebra is 1, that
# can save a shitload of time (especially over AA).
- powers = tuple( elt**k for k in range(superalgebra.dimension()) )
- power_vectors = ( p.to_vector() for p in powers )
- P = matrix(superalgebra.base_ring(), power_vectors)
-
- # Echelonize the matrix ourselves, because otherwise the
- # call to P.pivot_rows() below can choose a non-optimal
- # row-reduction algorithm. In particular, scaling can
- # help over AA because it avoids the RecursionError that
- # gets thrown when we have to look too hard for a root.
- #
- # Beware: QQ supports an entirely different set of "algorithm"
- # keywords than do AA and RR.
- algo = None
- if superalgebra.base_ring() is not QQ:
- algo = "scaled_partial_pivoting"
- P.echelonize(algorithm=algo)
-
- # Figure out which powers form a linearly-independent set.
- ind_rows = P.pivot_rows()
-
- # Pick those out of the list of all powers.
- basis = tuple(map(powers.__getitem__, ind_rows))
-
+ powers = tuple( elt**k for k in range(elt.degree()) )
super().__init__(superalgebra,
- basis,
+ powers,
associative=True,
**kwargs)
projects / sage.d.git / commitdiff