def __init__(self, R, names, index_set, **kwds):
"""
Initialize ``self``.
EXAMPLES::
sage: L = LieAlgebra(QQ, 3, 'x', abelian=True)
sage: TestSuite(L).run()
"""
LieAlgebraWithStructureCoefficients.__init__(self, R, Family({}), names, index_set, **kwds)
def __init__(self, R, names, index_set, **kwds):
"""
Initialize ``self``.
EXAMPLES::
sage: L = LieAlgebra(QQ, 3, 'x', abelian=True)
sage: TestSuite(L).run()
"""
LieAlgebraWithStructureCoefficients.__init__(self, R, Family({}),
names, index_set, **kwds)
def __init__(self, R, names, index_set, category, **kwds):
"""
Initialize ``self``.
EXAMPLES::
sage: L = LieAlgebra(QQ, 3, 'x', abelian=True)
sage: TestSuite(L).run()
"""
cat = LieAlgebras(R).FiniteDimensional().WithBasis().Nilpotent()
category = cat.or_subcategory(category)
LieAlgebraWithStructureCoefficients.__init__(self, R, Family({}), names,
index_set, category, **kwds)
def __init__(self, R, s_coeff, names, index_set, step=None, **kwds):
r"""
Initialize ``self``.
EXAMPLES::
sage: L.<X,Y,Z,W> = LieAlgebra(QQ, {('X','Y'): {'Z': 1}}, nilpotent=True)
sage: TestSuite(L).run()
"""
if step is not None:
self._step = step
LieAlgebraWithStructureCoefficients.__init__(self, R, s_coeff, names,
index_set, **kwds)
def __init__(self, R, s_coeff, names, index_set, step=None, **kwds):
r"""
Initialize ``self``.
EXAMPLES::
sage: L.<X,Y,Z,W> = LieAlgebra(QQ, {('X','Y'): {'Z': 1}}, nilpotent=True)
sage: TestSuite(L).run()
"""
if step is not None:
self._step = step
LieAlgebraWithStructureCoefficients.__init__(self, R, s_coeff,
names, index_set,
**kwds)
def __classcall_private__(cls,
R,
classification,
s_coeff,
names,
index_set=None,
category=None,
**kwds):
r"""
Normalize input to ensure a unique representation.
"""
names, index_set = standardize_names_index_set(names, index_set)
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
cat = LieAlgebras(R).FiniteDimensional().WithBasis().Nilpotent()
category = cat.or_subcategory(category)
return super(ClassifiedNilpotentLieAlgebra,
cls).__classcall__(cls,
R,
classification,
s_coeff,
names,
index_set,
category=category,
**kwds)
def __classcall_private__(cls, R, s_coeff, names=None, index_set=None,
category=None, **kwds):
"""
Normalize input to ensure a unique representation.
EXAMPLES:
If the variable order is specified, the order of structural
coefficients does not matter::
sage: from sage.algebras.lie_algebras.nilpotent_lie_algebra import NilpotentLieAlgebra_dense
sage: L1.<x,y,z> = NilpotentLieAlgebra_dense(QQ, {('x','y'): {'z': 1}})
sage: L2.<x,y,z> = NilpotentLieAlgebra_dense(QQ, {('y','x'): {'z': -1}})
sage: L1 is L2
True
If the variables are implicitly defined by the structural coefficients,
the ordering may be different and the Lie algebras will be considered
different::
sage: from sage.algebras.lie_algebras.nilpotent_lie_algebra import NilpotentLieAlgebra_dense
sage: L1 = NilpotentLieAlgebra_dense(QQ, {('x','y'): {'z': 1}})
sage: L2 = NilpotentLieAlgebra_dense(QQ, {('y','x'): {'z': -1}})
sage: L1
Nilpotent Lie algebra on 3 generators (x, y, z) over Rational Field
sage: L2
Nilpotent Lie algebra on 3 generators (y, x, z) over Rational Field
sage: L1 is L2
False
Constructed using two different methods from :class:`LieAlgebra`
yields the same Lie algebra::
sage: sc = {('X','Y'): {'Z': 1}}
sage: C = LieAlgebras(QQ).Nilpotent().FiniteDimensional().WithBasis()
sage: L1.<X,Y,Z> = LieAlgebra(QQ, sc, category=C)
sage: L2 = LieAlgebra(QQ, sc, nilpotent=True, names=['X','Y','Z'])
sage: L1 is L2
True
"""
if not names:
# extract names from structural coefficients
names = []
for (X, Y), d in s_coeff.items():
if X not in names: names.append(X)
if Y not in names: names.append(Y)
for k in d:
if k not in names: names.append(k)
from sage.structure.indexed_generators import standardize_names_index_set
names, index_set = standardize_names_index_set(names, index_set)
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
cat = LieAlgebras(R).FiniteDimensional().WithBasis().Nilpotent()
category = cat.or_subcategory(category)
return super(NilpotentLieAlgebra_dense, cls).__classcall__(
cls, R, s_coeff, names, index_set, category=category, **kwds)
def __init__(self, I, L, names, index_set, category=None):
r"""
Initialize ``self``.
TESTS::
sage: L.<x,y,z> = LieAlgebra(SR, {('x','y'): {'x':1}})
sage: K = L.quotient(y)
sage: K.dimension()
1
sage: TestSuite(K).run()
"""
B = L.basis()
sm = L.module().submodule_with_basis(
[I.reduce(B[i]).to_vector() for i in index_set])
SB = sm.basis()
# compute and normalize structural coefficients for the quotient
s_coeff = {}
for i, ind_i in enumerate(index_set):
for j in range(i + 1, len(index_set)):
ind_j = index_set[j]
brkt = I.reduce(L.bracket(SB[i], SB[j]))
brktvec = sm.coordinate_vector(brkt.to_vector())
s_coeff[(ind_i, ind_j)] = dict(zip(index_set, brktvec))
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
self._ambient = L
self._I = I
self._sm = sm
LieAlgebraWithStructureCoefficients.__init__(self,
L.base_ring(),
s_coeff,
names,
index_set,
category=category)
# register the quotient morphism as a conversion
H = Hom(L, self)
f = SetMorphism(H, self.retract)
self.register_conversion(f)
def __init__(self, I, L, names, index_set, category=None):
r"""
Initialize ``self``.
TESTS::
sage: L.<x,y,z> = LieAlgebra(SR, {('x','y'): {'x':1}})
sage: K = L.quotient(y)
sage: K.dimension()
1
sage: TestSuite(K).run()
"""
B = L.basis()
sm = L.module().submodule_with_basis([I.reduce(B[i]).to_vector()
for i in index_set])
SB = sm.basis()
# compute and normalize structural coefficients for the quotient
s_coeff = {}
for i, ind_i in enumerate(index_set):
for j in range(i + 1, len(index_set)):
ind_j = index_set[j]
brkt = I.reduce(L.bracket(SB[i], SB[j]))
brktvec = sm.coordinate_vector(brkt.to_vector())
s_coeff[(ind_i, ind_j)] = dict(zip(index_set, brktvec))
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
self._ambient = L
self._I = I
self._sm = sm
LieAlgebraWithStructureCoefficients.__init__(
self, L.base_ring(), s_coeff, names, index_set, category=category)
# register the quotient morphism as a conversion
H = Hom(L, self)
f = SetMorphism(H, self.retract)
self.register_conversion(f)
def affine_transformations_line(R, names=['X', 'Y'], representation='bracket'):
"""
The Lie algebra of affine transformations of the line.
EXAMPLES::
sage: L = lie_algebras.affine_transformations_line(QQ)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y}
sage: X, Y = L.lie_algebra_generators()
sage: L[X, Y] == Y
True
sage: TestSuite(L).run()
sage: L = lie_algebras.affine_transformations_line(QQ, representation="matrix")
sage: X, Y = L.lie_algebra_generators()
sage: L[X, Y] == Y
True
sage: TestSuite(L).run()
"""
if isinstance(names, str):
names = names.split(',')
names = tuple(names)
if representation == 'matrix':
from sage.matrix.matrix_space import MatrixSpace
MS = MatrixSpace(R, 2, sparse=True)
one = R.one()
gens = tuple(MS({(0, i): one}) for i in range(2))
from sage.algebras.lie_algebras.lie_algebra import LieAlgebraFromAssociative
return LieAlgebraFromAssociative(MS, gens, names=names)
X = names[0]
Y = names[1]
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
s_coeff = {(X, Y): {Y: R.one()}}
L = LieAlgebraWithStructureCoefficients(R, s_coeff, names=names)
L.rename(
"Lie algebra of affine transformations of a line over {}".format(R))
return L
def affine_transformations_line(R, names=['X', 'Y'], representation='bracket'):
"""
The Lie algebra of affine transformations of the line.
EXAMPLES::
sage: L = lie_algebras.affine_transformations_line(QQ)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y}
sage: X, Y = L.lie_algebra_generators()
sage: L[X, Y] == Y
True
sage: TestSuite(L).run()
sage: L = lie_algebras.affine_transformations_line(QQ, representation="matrix")
sage: X, Y = L.lie_algebra_generators()
sage: L[X, Y] == Y
True
sage: TestSuite(L).run()
"""
if isinstance(names, str):
names = names.split(',')
names = tuple(names)
if representation == 'matrix':
from sage.matrix.matrix_space import MatrixSpace
MS = MatrixSpace(R, 2, sparse=True)
one = R.one()
gens = tuple(MS({(0,i):one}) for i in range(2))
from sage.algebras.lie_algebras.lie_algebra import LieAlgebraFromAssociative
return LieAlgebraFromAssociative(MS, gens, names=names)
X = names[0]
Y = names[1]
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
s_coeff = {(X,Y): {Y:R.one()}}
L = LieAlgebraWithStructureCoefficients(R, s_coeff, names=names)
L.rename("Lie algebra of affine transformations of a line over {}".format(R))
return L
def three_dimensional(R, a, b, c, d, names=['X', 'Y', 'Z']):
r"""
The 3-dimensional Lie algebra over a given commutative ring `R`
with basis `\{X, Y, Z\}` subject to the relations:
.. MATH::
[X, Y] = aZ + dY, \quad [Y, Z] = bX, \quad [Z, X] = cY + dZ
where `a,b,c,d \in R`.
This is always a well-defined 3-dimensional Lie algebra, as can
be easily proven by computation.
EXAMPLES::
sage: L = lie_algebras.three_dimensional(QQ, 4, 1, -1, 2)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): 2*Y + 4*Z, ('X', 'Z'): Y - 2*Z, ('Y', 'Z'): X}
sage: TestSuite(L).run()
sage: L = lie_algebras.three_dimensional(QQ, 1, 0, 0, 0)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Z}
sage: L = lie_algebras.three_dimensional(QQ, 0, 0, -1, -1)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): -Y, ('X', 'Z'): Y + Z}
sage: L = lie_algebras.three_dimensional(QQ, 0, 1, 0, 0)
sage: L.structure_coefficients()
Finite family {('Y', 'Z'): X}
sage: lie_algebras.three_dimensional(QQ, 0, 0, 0, 0)
Abelian Lie algebra on 3 generators (X, Y, Z) over Rational Field
sage: Q.<a,b,c,d> = PolynomialRing(QQ)
sage: L = lie_algebras.three_dimensional(Q, a, b, c, d)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): d*Y + a*Z, ('X', 'Z'): (-c)*Y + (-d)*Z, ('Y', 'Z'): b*X}
sage: TestSuite(L).run()
"""
if isinstance(names, str):
names = names.split(',')
X = names[0]
Y = names[1]
Z = names[2]
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
s_coeff = {(X,Y): {Z:a, Y:d}, (Y,Z): {X:b}, (Z,X): {Y:c, Z:d}}
return LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
def _symbolic_lie_algebra_copy(L):
r"""
Create a copy of the Lie algebra ``L`` admitting symbolic coefficients.
This is used internally to compute a symbolic expression for the group law.
INPUT:
- ``L`` -- a finite dimensional Lie algebra with basis
EXAMPLES::
sage: from sage.groups.lie_gps.nilpotent_lie_group import _symbolic_lie_algebra_copy
sage: L = LieAlgebra(QQ, 2, step=2)
sage: L_SR = _symbolic_lie_algebra_copy(L)
sage: L.structure_coefficients()
Finite family {((1,), (2,)): X_12}
sage: L_SR.structure_coefficients()
Finite family {((1,), (2,)): L[(1, 2)]}
TESTS:
Verify that copying works with something that is not an instance of
:class:`LieAlgebraWithStructureCoefficients`::
sage: from sage.groups.lie_gps.nilpotent_lie_group import _symbolic_lie_algebra_copy
sage: L = lie_algebras.Heisenberg(QQ, 1)
sage: hasattr(L, 'change_ring')
False
sage: L_SR = _symbolic_lie_algebra_copy(L)
sage: L_SR.structure_coefficients()
Finite family {('p1', 'q1'): z}
"""
try:
return L.change_ring(SR)
except AttributeError:
s_coeff = L.structure_coefficients()
index_set = L.basis().keys()
names = L.variable_names()
return LieAlgebraWithStructureCoefficients(SR,
s_coeff,
names=names,
index_set=index_set)
def __classcall_private__(cls,
R,
s_coeff,
names=None,
index_set=None,
category=None,
**kwds):
"""
Normalize input to ensure a unique representation.
EXAMPLES:
If the variable order is specified, the order of structural
coefficients does not matter::
sage: from sage.algebras.lie_algebras.nilpotent_lie_algebra import NilpotentLieAlgebra_dense
sage: L1.<x,y,z> = NilpotentLieAlgebra_dense(QQ, {('x','y'): {'z': 1}})
sage: L2.<x,y,z> = NilpotentLieAlgebra_dense(QQ, {('y','x'): {'z': -1}})
sage: L1 is L2
True
If the variables are implicitly defined by the structural coefficients,
the ordering may be different and the Lie algebras will be considered
different::
sage: from sage.algebras.lie_algebras.nilpotent_lie_algebra import NilpotentLieAlgebra_dense
sage: L1 = NilpotentLieAlgebra_dense(QQ, {('x','y'): {'z': 1}})
sage: L2 = NilpotentLieAlgebra_dense(QQ, {('y','x'): {'z': -1}})
sage: L1
Nilpotent Lie algebra on 3 generators (x, y, z) over Rational Field
sage: L2
Nilpotent Lie algebra on 3 generators (y, x, z) over Rational Field
sage: L1 is L2
False
Constructed using two different methods from :class:`LieAlgebra`
yields the same Lie algebra::
sage: sc = {('X','Y'): {'Z': 1}}
sage: C = LieAlgebras(QQ).Nilpotent().FiniteDimensional().WithBasis()
sage: L1.<X,Y,Z> = LieAlgebra(QQ, sc, category=C)
sage: L2 = LieAlgebra(QQ, sc, nilpotent=True, names=['X','Y','Z'])
sage: L1 is L2
True
"""
if not names:
# extract names from structural coefficients
names = []
for (X, Y), d in s_coeff.items():
if X not in names: names.append(X)
if Y not in names: names.append(Y)
for k in d:
if k not in names: names.append(k)
from sage.structure.indexed_generators import standardize_names_index_set
names, index_set = standardize_names_index_set(names, index_set)
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
cat = LieAlgebras(R).FiniteDimensional().WithBasis().Nilpotent()
category = cat.or_subcategory(category)
return super(NilpotentLieAlgebra_dense,
cls).__classcall__(cls,
R,
s_coeff,
names,
index_set,
category=category,
**kwds)
def __init__(self, R, r, s, names, naming, category, **kwds):
r"""
Initialize ``self``
EXAMPLES::
sage: L = LieAlgebra(ZZ, 2, step=2)
sage: TestSuite(L).run()
sage: L = LieAlgebra(QQ, 4, step=3)
sage: TestSuite(L).run() # long time
"""
if r not in ZZ or r <= 0:
raise ValueError("number of generators %s is not "
"a positive integer" % r)
if s not in ZZ or s <= 0:
raise ValueError("step %s is not a positive integer" % s)
# extract an index set from the Lyndon words of the corresponding
# free Lie algebra, and store the corresponding elements in a dict
from sage.algebras.lie_algebras.lie_algebra import LieAlgebra
free_gen_names = ['F%d' % k for k in range(r)]
free_gen_names_inv = {
val: i + 1
for i, val in enumerate(free_gen_names)
}
L = LieAlgebra(R, free_gen_names).Lyndon()
basis_by_deg = {d: [] for d in range(1, s + 1)}
for d in range(1, s + 1):
for X in L.graded_basis(d):
# convert brackets of form [X_1, [X_1, X_2]] to words (1,1,2)
w = tuple(free_gen_names_inv[s]
for s in X.leading_support().to_word())
basis_by_deg[d].append((w, X))
index_set = [ind for d in basis_by_deg for ind, val in basis_by_deg[d]]
if len(names) == 1 and len(index_set) > 1:
if not naming:
if r >= 10:
naming = 'linear'
else:
naming = 'index'
if naming == 'linear':
names = [
'%s_%d' % (names[0], k + 1) for k in range(len(index_set))
]
elif naming == 'index':
if r >= 10:
raise ValueError("'index' naming scheme not supported for "
"10 or more generators")
names = [
'%s_%s' % (names[0], "".join(str(s) for s in ind))
for ind in index_set
]
else:
raise ValueError("unknown naming scheme %s" % naming)
# extract structural coefficients from the free Lie algebra
s_coeff = {}
for dx in range(1, s + 1):
# Brackets are only computed when deg(X) + deg(Y) <= s
# We also require deg(Y) >= deg(X) by the ordering
for dy in range(dx, s + 1 - dx):
if dx == dy:
for i, val in enumerate(basis_by_deg[dx]):
X_ind, X = val
for Y_ind, Y in basis_by_deg[dy][i + 1:]:
Z = L[X, Y]
if not Z.is_zero():
s_coeff[(X_ind, Y_ind)] = {
W_ind: Z[W.leading_support()]
for W_ind, W in basis_by_deg[dx + dy]
}
else:
for X_ind, X in basis_by_deg[dx]:
for Y_ind, Y in basis_by_deg[dy]:
Z = L[X, Y]
if not Z.is_zero():
s_coeff[(X_ind, Y_ind)] = {
W_ind: Z[W.leading_support()]
for W_ind, W in basis_by_deg[dx + dy]
}
names, index_set = standardize_names_index_set(names, index_set)
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
NilpotentLieAlgebra_dense.__init__(self,
R,
s_coeff,
names,
index_set,
s,
category=category,
**kwds)
def __init__(self, R, cartan_type):
r"""
Initialize ``self``.
TESTS::
sage: L = LieAlgebra(QQ, cartan_type=['A',2])
sage: TestSuite(L).run() # long time
"""
self._cartan_type = cartan_type
RL = cartan_type.root_system().root_lattice()
alpha = RL.simple_roots()
p_roots = list(RL.positive_roots_by_height())
n_roots = [-x for x in p_roots]
self._p_roots_index = {al: i for i,al in enumerate(p_roots)}
alphacheck = RL.simple_coroots()
roots = frozenset(RL.roots())
num_sroots = len(alpha)
one = R.one()
# Determine the signs for the structure coefficients from the root system
# We first create the special roots
sp_sign = {}
for i,a in enumerate(p_roots):
for b in p_roots[i+1:]:
if a + b not in p_roots:
continue
# Compute the sign for the extra special pair
x, y = (a + b).extraspecial_pair()
if (x, y) == (a, b): # If it already is an extra special pair
if (x, y) not in sp_sign:
# This swap is so the structure coefficients match with GAP
if (sum(x.coefficients()) == sum(y.coefficients())
and str(x) > str(y)):
y,x = x,y
sp_sign[(x, y)] = -one
sp_sign[(y, x)] = one
continue
if b - x in roots:
t1 = ((b-x).norm_squared() / b.norm_squared()
* sp_sign[(x, b-x)] * sp_sign[(a, y-a)])
else:
t1 = 0
if a - x in roots:
t2 = ((a-x).norm_squared() / a.norm_squared()
* sp_sign[(x, a-x)] * sp_sign[(b, y-b)])
else:
t2 = 0
if t1 - t2 > 0:
sp_sign[(a,b)] = -one
elif t2 - t1 > 0:
sp_sign[(a,b)] = one
sp_sign[(b,a)] = -sp_sign[(a,b)]
# Function to construct the structure coefficients (up to sign)
def e_coeff(r, s):
p = 1
while r - p*s in roots:
p += 1
return p
# Now we can compute all necessary structure coefficients
s_coeffs = {}
for i,r in enumerate(p_roots):
# [e_r, h_i] and [h_i, f_r]
for ac in alphacheck:
c = r.scalar(ac)
if c == 0:
continue
s_coeffs[(r, ac)] = {r: -c}
s_coeffs[(ac, -r)] = {-r: -c}
# [e_r, f_r]
s_coeffs[(r, -r)] = {alphacheck[j]: c
for j, c in r.associated_coroot()}
# [e_r, e_s] and [e_r, f_s] with r != +/-s
# We assume s is positive, as otherwise we negate
# both r and s and the resulting coefficient
for j, s in enumerate(p_roots[i+1:]):
j += i+1 # Offset
# Since h(s) >= h(r), we have s - r > 0 when s - r is a root
# [f_r, e_s]
if s - r in p_roots:
c = e_coeff(r, -s)
a, b = s-r, r
if self._p_roots_index[a] > self._p_roots_index[b]: # Note a != b
c *= -sp_sign[(b, a)]
else:
c *= sp_sign[(a, b)]
s_coeffs[(-r, s)] = {a: -c}
s_coeffs[(r, -s)] = {-a: c}
# [e_r, e_s]
a = r + s
if a in p_roots:
# (r, s) is a special pair
c = e_coeff(r, s) * sp_sign[(r, s)]
s_coeffs[(r, s)] = {a: c}
s_coeffs[(-r, -s)] = {-a: -c}
# Lastly, make sure a < b for all (a, b) in the coefficients and flip if necessary
for k in s_coeffs.keys():
a,b = k[0], k[1]
if self._basis_key(a) > self._basis_key(b):
s_coeffs[(b,a)] = [(index, -v) for index,v in s_coeffs[k].items()]
del s_coeffs[k]
else:
s_coeffs[k] = s_coeffs[k].items()
names = ['e{}'.format(i) for i in range(1, num_sroots+1)]
names += ['f{}'.format(i) for i in range(1, num_sroots+1)]
names += ['h{}'.format(i) for i in range(1, num_sroots+1)]
category = LieAlgebras(R).FiniteDimensional().WithBasis()
index_set = p_roots + list(alphacheck) + n_roots
names = tuple(names)
from sage.sets.finite_enumerated_set import FiniteEnumeratedSet
index_set = FiniteEnumeratedSet(index_set)
LieAlgebraWithStructureCoefficients.__init__(self, R, s_coeffs, names, index_set,
category, prefix='E', bracket='[',
sorting_key=self._basis_key)
def three_dimensional_by_rank(R, n, a=None, names=['X', 'Y', 'Z']):
"""
Return a 3-dimensional Lie algebra of rank ``n``, where `0 \leq n \leq 3`.
Here, the *rank* of a Lie algebra `L` is defined as the dimension
of its derived subalgebra `[L, L]`. (We are assuming that `R` is
a field of characteristic `0`; otherwise the Lie algebras
constructed by this function are still well-defined but no longer
might have the correct ranks.) This is not to be confused with
the other standard definition of a rank (namely, as the
dimension of a Cartan subalgebra, when `L` is semisimple).
INPUT:
- ``R`` -- the base ring
- ``n`` -- the rank
- ``a`` -- the deformation parameter (used for `n = 2`); this should
be a nonzero element of `R` in order for the resulting Lie
algebra to actually have the right rank(?)
- ``names`` -- (optional) the generator names
EXAMPLES::
sage: lie_algebras.three_dimensional_by_rank(QQ, 0)
Abelian Lie algebra on 3 generators (X, Y, Z) over Rational Field
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 1)
sage: L.structure_coefficients()
Finite family {('Y', 'Z'): X}
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 2, 4)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y, ('X', 'Z'): Y + Z}
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 2, 0)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y}
sage: lie_algebras.three_dimensional_by_rank(QQ, 3)
sl2 over Rational Field
"""
if isinstance(names, str):
names = names.split(',')
names = tuple(names)
if n == 0:
from sage.algebras.lie_algebras.abelian import AbelianLieAlgebra
return AbelianLieAlgebra(R, names=names)
if n == 1:
L = three_dimensional(R, 0, 1, 0, 0, names=names) # Strictly upper triangular matrices
L.rename("Lie algebra of 3x3 strictly upper triangular matrices over {}".format(R))
return L
if n == 2:
if a is None:
raise ValueError("The parameter 'a' must be specified")
X = names[0]
Y = names[1]
Z = names[2]
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
if a == 0:
s_coeff = {(X,Y): {Y:R.one()}, (X,Z): {Y:R(a)}}
# Why use R(a) here if R == 0 ? Also this has rank 1.
L = LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
L.rename("Degenerate Lie algebra of dimension 3 and rank 2 over {}".format(R))
else:
s_coeff = {(X,Y): {Y:R.one()}, (X,Z): {Y:R.one(), Z:R.one()}}
# a doesn't appear here :/
L = LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
L.rename("Lie algebra of dimension 3 and rank 2 with parameter {} over {}".format(a, R))
return L
if n == 3:
#return sl(R, 2)
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
E = names[0]
F = names[1]
H = names[2]
s_coeff = { (E,F): {H:R.one()}, (H,E): {E:R(2)}, (H,F): {F:R(-2)} }
L = LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
L.rename("sl2 over {}".format(R))
return L
raise ValueError("Invalid rank")
def __init__(self, R, r, s, names, naming, category, **kwds):
r"""
Initialize ``self``
EXAMPLES::
sage: L = LieAlgebra(ZZ, 2, step=2)
sage: TestSuite(L).run()
sage: L = LieAlgebra(QQ, 4, step=3)
sage: TestSuite(L).run() # long time
"""
if r not in ZZ or r <= 0:
raise ValueError("number of generators %s is not "
"a positive integer" % r)
if s not in ZZ or s <= 0:
raise ValueError("step %s is not a positive integer" % s)
# extract an index set from the Lyndon words of the corresponding
# free Lie algebra, and store the corresponding elements in a dict
from sage.algebras.lie_algebras.lie_algebra import LieAlgebra
free_gen_names = ['F%d' % k for k in range(r)]
free_gen_names_inv = {val: i + 1 for i, val in enumerate(free_gen_names)}
L = LieAlgebra(R, free_gen_names).Lyndon()
basis_by_deg = {d: [] for d in range(1, s + 1)}
for d in range(1, s + 1):
for X in L.graded_basis(d):
# convert brackets of form [X_1, [X_1, X_2]] to words (1,1,2)
w = tuple(free_gen_names_inv[s]
for s in X.leading_support().to_word())
basis_by_deg[d].append((w, X))
index_set = [ind for d in basis_by_deg for ind, val in basis_by_deg[d]]
if len(names) == 1 and len(index_set) > 1:
if not naming:
if r >= 10:
naming = 'linear'
else:
naming = 'index'
if naming == 'linear':
names = ['%s_%d' % (names[0], k + 1)
for k in range(len(index_set))]
elif naming == 'index':
if r >= 10:
raise ValueError("'index' naming scheme not supported for "
"10 or more generators")
names = ['%s_%s' % (names[0], "".join(str(s) for s in w))
for w in index_set]
else:
raise ValueError("unknown naming scheme %s" % naming)
# extract structural coefficients from the free Lie algebra
s_coeff = {}
for dx in range(1, s + 1):
# Brackets are only computed when deg(X) + deg(Y) <= s
# We also require deg(Y) >= deg(X) by the ordering
for dy in range(dx, s + 1 - dx):
if dx == dy:
for i, val in enumerate(basis_by_deg[dx]):
X_ind, X = val
for Y_ind, Y in basis_by_deg[dy][i + 1:]:
Z = L[X, Y]
if not Z.is_zero():
s_coeff[(X_ind, Y_ind)] = {W_ind: Z[W.leading_support()]
for W_ind, W in basis_by_deg[dx + dy]}
else:
for X_ind, X in basis_by_deg[dx]:
for Y_ind, Y in basis_by_deg[dy]:
Z = L[X, Y]
if not Z.is_zero():
s_coeff[(X_ind, Y_ind)] = {W_ind: Z[W.leading_support()]
for W_ind, W in basis_by_deg[dx + dy]}
names, index_set = standardize_names_index_set(names, index_set)
s_coeff = LieAlgebraWithStructureCoefficients._standardize_s_coeff(
s_coeff, index_set)
NilpotentLieAlgebra_dense.__init__(self, R, s_coeff, names,
index_set, s,
category=category, **kwds)
def three_dimensional_by_rank(R, n, a=None, names=['X', 'Y', 'Z']):
r"""
Return a 3-dimensional Lie algebra of rank ``n``, where `0 \leq n \leq 3`.
Here, the *rank* of a Lie algebra `L` is defined as the dimension
of its derived subalgebra `[L, L]`. (We are assuming that `R` is
a field of characteristic `0`; otherwise the Lie algebras
constructed by this function are still well-defined but no longer
might have the correct ranks.) This is not to be confused with
the other standard definition of a rank (namely, as the
dimension of a Cartan subalgebra, when `L` is semisimple).
INPUT:
- ``R`` -- the base ring
- ``n`` -- the rank
- ``a`` -- the deformation parameter (used for `n = 2`); this should
be a nonzero element of `R` in order for the resulting Lie
algebra to actually have the right rank(?)
- ``names`` -- (optional) the generator names
EXAMPLES::
sage: lie_algebras.three_dimensional_by_rank(QQ, 0)
Abelian Lie algebra on 3 generators (X, Y, Z) over Rational Field
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 1)
sage: L.structure_coefficients()
Finite family {('Y', 'Z'): X}
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 2, 4)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y, ('X', 'Z'): Y + Z}
sage: L = lie_algebras.three_dimensional_by_rank(QQ, 2, 0)
sage: L.structure_coefficients()
Finite family {('X', 'Y'): Y}
sage: lie_algebras.three_dimensional_by_rank(QQ, 3)
sl2 over Rational Field
"""
if isinstance(names, str):
names = names.split(',')
names = tuple(names)
if n == 0:
from sage.algebras.lie_algebras.abelian import AbelianLieAlgebra
return AbelianLieAlgebra(R, names=names)
if n == 1:
L = three_dimensional(R, 0, 1, 0, 0, names=names) # Strictly upper triangular matrices
L.rename("Lie algebra of 3x3 strictly upper triangular matrices over {}".format(R))
return L
if n == 2:
if a is None:
raise ValueError("The parameter 'a' must be specified")
X = names[0]
Y = names[1]
Z = names[2]
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
if a == 0:
s_coeff = {(X,Y): {Y:R.one()}, (X,Z): {Y:R(a)}}
# Why use R(a) here if R == 0 ? Also this has rank 1.
L = LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
L.rename("Degenerate Lie algebra of dimension 3 and rank 2 over {}".format(R))
else:
s_coeff = {(X,Y): {Y:R.one()}, (X,Z): {Y:R.one(), Z:R.one()}}
# a doesn't appear here :/
L = LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
L.rename("Lie algebra of dimension 3 and rank 2 with parameter {} over {}".format(a, R))
return L
if n == 3:
#return sl(R, 2)
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
E = names[0]
F = names[1]
H = names[2]
s_coeff = { (E,F): {H:R.one()}, (H,E): {E:R(2)}, (H,F): {F:R(-2)} }
L = LieAlgebraWithStructureCoefficients(R, s_coeff, tuple(names))
L.rename("sl2 over {}".format(R))
return L
raise ValueError("Invalid rank")
def __classcall_private__(cls,
R=None,
arg0=None,
arg1=None,
names=None,
index_set=None,
abelian=False,
**kwds):
"""
Select the correct parent based upon input.
TESTS::
sage: LieAlgebra(QQ, abelian=True, names='x,y,z')
Abelian Lie algebra on 3 generators (x, y, z) over Rational Field
sage: LieAlgebra(QQ, {('e','h'): {'e':-2}, ('f','h'): {'f':2},
....: ('e','f'): {'h':1}}, names='e,f,h')
Lie algebra on 3 generators (e, f, h) over Rational Field
"""
# Parse associative algebra input
# -----
assoc = kwds.get("associative", None)
if assoc is not None:
return LieAlgebraFromAssociative(assoc,
names=names,
index_set=index_set)
# Parse input as a Cartan type
# -----
ct = kwds.get("cartan_type", None)
if ct is not None:
from sage.combinat.root_system.cartan_type import CartanType
ct = CartanType(ct)
if ct.is_affine():
from sage.algebras.lie_algebras.affine_lie_algebra import AffineLieAlgebra
return AffineLieAlgebra(R,
cartan_type=ct,
kac_moody=kwds.get("kac_moody", True))
if not ct.is_finite():
raise NotImplementedError(
"non-finite types are not implemented yet, see trac #14901 for details"
)
rep = kwds.get("representation", "bracket")
if rep == 'bracket':
from sage.algebras.lie_algebras.classical_lie_algebra import LieAlgebraChevalleyBasis
return LieAlgebraChevalleyBasis(R, ct)
if rep == 'matrix':
from sage.algebras.lie_algebras.classical_lie_algebra import ClassicalMatrixLieAlgebra
return ClassicalMatrixLieAlgebra(R, ct)
raise ValueError("invalid representation")
# Parse the remaining arguments
# -----
if R is None:
raise ValueError("invalid arguments")
check_assoc = lambda A: (isinstance(A, (Ring, MatrixSpace)) or A in
Rings() or A in Algebras(R).Associative())
if arg0 in ZZ or check_assoc(arg1):
# Check if we need to swap the arguments
arg0, arg1 = arg1, arg0
# Parse the first argument
# -----
if isinstance(arg0, dict):
if not arg0:
from sage.algebras.lie_algebras.abelian import AbelianLieAlgebra
return AbelianLieAlgebra(R, names, index_set)
elif isinstance(next(iter(arg0.keys())), (list, tuple)):
# We assume it is some structure coefficients
arg1, arg0 = arg0, arg1
if isinstance(arg0, (list, tuple)):
if all(isinstance(x, str) for x in arg0):
# If they are all strings, then it is a list of variables
names = tuple(arg0)
if isinstance(arg0, str):
names = tuple(arg0.split(','))
elif isinstance(names, str):
names = tuple(names.split(','))
# Parse the second argument
if isinstance(arg1, dict):
# Assume it is some structure coefficients
from sage.algebras.lie_algebras.structure_coefficients import LieAlgebraWithStructureCoefficients
return LieAlgebraWithStructureCoefficients(R, arg1, names,
index_set, **kwds)
# Otherwise it must be either a free or abelian Lie algebra
if arg1 in ZZ:
if isinstance(arg0, str):
names = arg0
if names is None:
index_set = list(range(arg1))
else:
if isinstance(names, str):
names = tuple(names.split(','))
if arg1 != 1 and len(names) == 1:
names = tuple('{}{}'.format(names[0], i)
for i in range(arg1))
if arg1 != len(names):
raise ValueError("the number of names must equal the"
" number of generators")
if abelian:
from sage.algebras.lie_algebras.abelian import AbelianLieAlgebra
return AbelianLieAlgebra(R, names, index_set)
# Otherwise it is the free Lie algebra
rep = kwds.get("representation", "bracket")
if rep == "polynomial":
# Construct the free Lie algebra from polynomials in the
# free (associative unital) algebra
# TODO: Change this to accept an index set once FreeAlgebra accepts one
from sage.algebras.free_algebra import FreeAlgebra
F = FreeAlgebra(R, names)
if index_set is None:
index_set = F.variable_names()
# TODO: As part of #16823, this should instead construct a
# subclass with specialized methods for the free Lie algebra
return LieAlgebraFromAssociative(F,
F.gens(),
names=names,
index_set=index_set)
raise NotImplementedError("the free Lie algebra has only been"
" implemented using polynomials in the"
" free algebra, see trac ticket #16823")
