"""Module exposing the GeneralLinear group class."""
from itertools import product
import geomstats.backend as gs
from geomstats.geometry.matrices import Matrices
[docs]class GeneralLinear(Matrices):
"""Class for the general linear group GL(n).
Parameters
----------
n : int
Integer representing the shape of the matrices: n x n.
"""
def __init__(self, n, **kwargs):
Matrices.__init__(self, n, n)
self.n = n
[docs] def belongs(self, point):
"""Check if a matrix is invertible and of the right shape.
Parameters
----------
point : array-like, shape=[..., n, n]
Matrix to be checked.
Returns
-------
belongs : array-like, shape=[...,]
Boolean denoting if point is in GL(n).
"""
point_shape = point.shape
mat_dim_1, mat_dim_2 = point_shape[-2], point_shape[-1]
det = gs.linalg.det(point)
return gs.logical_and(
mat_dim_1 == self.n and mat_dim_2 == self.n,
det != 0.)
[docs] def get_identity(self):
"""Return the identity matrix."""
return gs.eye(self.n, self.n)
identity = property(get_identity)
[docs] @classmethod
def compose(cls, *args):
"""Return the product of a collection of matrices."""
return cls.mul(*args)
[docs] @staticmethod
def inverse(point):
"""Return the inverse of a matrix.
Parameters
----------
point : array-like, shape=[..., n, n]
Matrix to be inverted.
"""
return gs.linalg.inv(point)
def _replace_values(self, samples, new_samples, indcs):
"""Replace samples with new samples at specific indices."""
replaced_indices = [
i for i, is_replaced in enumerate(indcs) if is_replaced]
value_indices = list(
product(replaced_indices, range(self.n), range(self.n)))
return gs.assignment(samples, gs.flatten(new_samples), value_indices)
[docs] @classmethod
def exp(cls, tangent_vec, base_point=None):
r"""
Exponentiate a left-invariant vector field from a base point.
The vector input is not an element of the Lie algebra, but of the
tangent space at base_point: if :math:`g` denotes `base_point`,
:math:`v` the tangent vector, and :math:'V = g^{-1} v' the associated
Lie algebra vector, then
.. math::
\exp(v, g) = mul(g, \exp(V))
Therefore, the Lie exponential is obtained when base_point is None, or
the identity.
Parameters
----------
tangent_vec : array-like, shape=[..., n, n]
Tangent vector at base point.
base_point : array-like, shape=[..., n, n]
Base point.
Optional, defaults to identity if None.
Returns
-------
point : array-like, shape=[..., n, n]
Left multiplication of `exp(algebra_mat)` with `base_point`.
"""
expm = gs.linalg.expm
if base_point is None:
return expm(tangent_vec)
lie_algebra_vec = cls.mul(cls.inverse(base_point), tangent_vec)
return cls.mul(base_point, cls.exp(lie_algebra_vec))
[docs] @classmethod
def log(cls, point, base_point=None):
r"""
Compute a left-invariant vector field bringing base_point to point.
The output is a vector of the tangent space at base_point, so not a Lie
algebra element if it is not the identity.
Parameters
----------
point : array-like, shape=[..., n, n]
Point.
base_point : array-like, shape=[..., n, n]
Base point.
Optional, defaults to identity if None.
Returns
-------
tangent_vec : array-like, shape=[..., n, n]
Matrix such that `exp(tangent_vec, base_point) = point`.
Notes
-----
Denoting `point` by :math:`g` and `base_point` by :math:`h`,
the output satisfies:
.. math::
g = \exp(\log(g, h), h)
"""
logm = gs.linalg.logm
if base_point is None:
return logm(point)
lie_algebra_vec = logm(cls.mul(cls.inverse(base_point), point))
return cls.mul(base_point, lie_algebra_vec)
[docs] @classmethod
def orbit(cls, point, base_point=None):
r"""
Compute the one-parameter orbit of base_point passing through point.
Parameters
----------
point : array-like, shape=[n, n]
Target point.
base_point : array-like, shape=[n, n], optional
Base point.
Optional, defaults to identity if None.
Returns
-------
path : callable
One-parameter orbit.
Satisfies `path(0) = base_point` and `path(1) = point`.
Notes
-----
Denoting `point` by :math:`g` and `base_point` by :math:`h`,
the orbit :math:`\gamma` satisfies:
.. math::
\gamma(t) = {\mathrm e}^{t X} \cdot h \\
\quad {\mathrm with} \quad\\
{\mathrm e}^{X} = g h^{-1}
The path is not uniquely defined and depends on the choice of :math:`V`
returned by :py:meth:`log`.
Vectorization
-------------
Return a collection of trajectories (4-D array)
from a collection of input matrices (3-D array).
"""
# TODO (nina): Will work when expm gets properly 4-D vectorized.
tangent_vec = cls.log(point, base_point)
def path(time):
vecs = gs.einsum('t,...ij->...tij', time, tangent_vec)
return cls.exp(vecs, base_point)
return path