Skip to content

Commit

Permalink
gh-36171: Implement Chevalley-Eilenberg complexes with module coeffic…
Browse files Browse the repository at this point in the history 
…ients

    
<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->

We implement the Chevalley-Eilenberg complex of a Lie algebra `L` with
coefficients in a general `L` representation.

<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

- #36169 uses the representations of Lie algebras

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #36171
Reported by: Travis Scrimshaw
Reviewer(s): adityadwarkesh
  • Loading branch information
Release Manager committed Feb 11, 2024
2 parents 2369ddb + adbccdc commit 5a3f8eb
Showing 1 changed file with 153 additions and 55 deletions.
208 changes: 153 additions & 55 deletions src/sage/categories/finite_dimensional_lie_algebras_with_basis.py
View file
Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,31 @@
from sage.sets.family import Family


def _ce_complex_key(self, M, d, s, n):
"""
The key for caching the Chevalley-Eilenberg complex.
TESTS::
sage: from sage.categories.finite_dimensional_lie_algebras_with_basis import _ce_complex_key
sage: L.<x,y> = LieAlgebra(QQ, {('x','y'): {'y':1}})
sage: _ce_complex_key(L, None, False, True, 5)
(None, False, True)
sage: f = ({x: Matrix([[1,0],[0,0]]), y: Matrix([[0,1],[0,0]])})
sage: _ce_complex_key(L, f, False, True, 5)
(Representation of Lie algebra on 2 generators (x, y) over Rational Field defined by:
[1 0]
x |--> [0 0]
[0 1]
y |--> [0 0],
False,
True)
"""
if isinstance(M, dict):
M = self.representation(M)
return (M, d, s)


class FiniteDimensionalLieAlgebrasWithBasis(CategoryWithAxiom_over_base_ring):
"""
Category of finite dimensional Lie algebras with a basis.
Expand Down Expand Up @@ -1103,7 +1128,7 @@ def is_semisimple(self):
"""
return not self.killing_form_matrix().is_singular()

@cached_method(key=lambda self,M,d,s,n: (M,d,s))
@cached_method(key=_ce_complex_key)
def chevalley_eilenberg_complex(self, M=None, dual=False, sparse=True, ncpus=None):
r"""
Return the Chevalley-Eilenberg complex of ``self``.
Expand Down Expand Up @@ -1134,7 +1159,13 @@ def chevalley_eilenberg_complex(self, M=None, dual=False, sparse=True, ncpus=Non
INPUT:
- ``M`` -- (default: the trivial 1-dimensional module)
the module `M`
one of the following:
* a module `M` with an action of ``self``
* a dictionary whose keys are basis elements and values
are matrices representing a Lie algebra homomorphism
defining the representation
- ``dual`` -- (default: ``False``) if ``True``, causes
the dual of the complex to be computed
- ``sparse`` -- (default: ``True``) whether to use sparse
Expand All @@ -1148,63 +1179,79 @@ def chevalley_eilenberg_complex(self, M=None, dual=False, sparse=True, ncpus=Non
sage: C = L.chevalley_eilenberg_complex(); C
Chain complex with at most 4 nonzero terms over Integer Ring
sage: ascii_art(C)
[ 2 0 0] [0]
[ 0 -1 0] [0]
[0 0 0] [ 0 0 2] [0]
[-2 0 0] [0]
[ 0 1 0] [0]
[0 0 0] [ 0 0 -2] [0]
0 <-- C_0 <-------- C_1 <----------- C_2 <---- C_3 <-- 0
sage: # long time, needs sage.combinat sage.modules
sage: # needs sage.combinat sage.modules
sage: L = LieAlgebra(QQ, cartan_type=['C',2])
sage: C = L.chevalley_eilenberg_complex()
sage: [C.free_module_rank(i) for i in range(11)]
sage: C = L.chevalley_eilenberg_complex() # long time
sage: [C.free_module_rank(i) for i in range(11)] # long time
[1, 10, 45, 120, 210, 252, 210, 120, 45, 10, 1]
sage: # needs sage.combinat sage.modules
sage: g = lie_algebras.sl(QQ, 2)
sage: E, F, H = g.basis()
sage: n = g.subalgebra([F, H])
sage: ascii_art(n.chevalley_eilenberg_complex())
[0]
[0 0] [2]
0 <-- C_0 <------ C_1 <---- C_2 <-- 0
[ 0]
[0 0] [-2]
0 <-- C_0 <------ C_1 <----- C_2 <-- 0
sage: L.<x,y> = LieAlgebra(QQ, {('x','y'): {'y':1}})
sage: f = ({x: Matrix([[1,0],[0,0]]), y: Matrix([[0,1],[0,0]])})
sage: C = L.chevalley_eilenberg_complex(f); C
Chain complex with at most 3 nonzero terms over Rational Field
sage: ascii_art(C)
[ 0 -1]
[ 2 0]
[1 0 0 1] [ 0 0]
[0 0 0 0] [ 0 1]
0 <-- C_0 <---------- C_1 <-------- C_2 <-- 0
sage: ascii_art(L.chevalley_eilenberg_complex(f, sparse=False))
[ 0 -1]
[ 2 0]
[1 0 0 1] [ 0 0]
[0 0 0 0] [ 0 1]
0 <-- C_0 <---------- C_1 <-------- C_2 <-- 0
REFERENCES:
- :wikipedia:`Lie_algebra_cohomology#Chevalley-Eilenberg_complex`
- [Wei1994]_ Chapter 7
.. TODO::
Currently this is only implemented for coefficients
given by the trivial module `R`, where `R` is the
base ring and `g R = 0` for all `g \in \mathfrak{g}`.
Allow generic coefficient modules `M`.
"""
if dual:
return self.chevalley_eilenberg_complex(M, dual=False,
sparse=sparse,
ncpus=ncpus).dual()

if M is not None:
raise NotImplementedError("only implemented for the default"
" (the trivial module)")

from itertools import combinations
import itertools
from itertools import combinations, product
from sage.arith.misc import binomial
from sage.matrix.matrix_space import MatrixSpace
from sage.algebras.lie_algebras.representation import Representation_abstract
R = self.base_ring()
zero = R.zero()
mone = -R.one()
if M is not None:
raise NotImplementedError("coefficient module M cannot be passed")

# Make sure we specify the ordering of the basis
B = self.basis()
K = list(B.keys())
B = [B[k] for k in K]
Ind = list(range(len(K)))
M = self.module()
ambient = M.is_ambient()
LB = self.basis()
LK = list(LB.keys())
LB = [LB[k] for k in LK]
LI = list(range(len(LK)))
Lmod = self.module()
ambient = Lmod.is_ambient()

if M is not None:
if not isinstance(M, Representation_abstract):
M = self.representation(M)

MB = M.basis()
MK = list(MB.keys())
MB = [MB[k] for k in MK]
MI = list(range(len(MK)))

def sgn(k, X):
"""
Expand All @@ -1231,69 +1278,120 @@ def sgn(k, X):
return R.one(), tuple(Y)

from sage.parallel.decorate import parallel
from sage.matrix.constructor import matrix

@parallel(ncpus=ncpus)
def compute_diff(k):
"""
Build the ``k``-th differential (in parallel).
"""
indices = {tuple(X): i for i,X in enumerate(combinations(Ind, k-1))}
# The indices for the exterior algebra
ext_ind = {tuple(X): i for i, X in enumerate(combinations(LI, k-1))}

# Compute the part independent of the module first ("part 2" of the computation)
if sparse:
data = {}
p2_data = {}
row = 0
else:
data = []
p2_data = []
if not sparse:
zero = [zero] * len(indices)
for X in combinations(Ind, k):
zv = [zero] * len(ext_ind)
for X in combinations(LI, k):
if not sparse:
ret = list(zero)
ret = list(zv)
for i in range(k):
Y = list(X)
Y.pop(i)
# We do mone**i because we are 0-based
# This is where we would do the action on
# the coefficients module
#ret[indices[tuple(Y)]] += mone**i * zero
for j in range(i+1,k):
# We shift j by 1 because we already removed
# an earlier element from X.
Z = tuple(Y[:j-1] + Y[j:])
elt = mone**(i+j) * B[X[i]].bracket(B[X[j]])
elt = mone**(i+j+1) * LB[X[i]].bracket(LB[X[j]])
if not elt:
continue
if ambient:
vec = elt.to_vector()
else:
vec = M.coordinate_vector(elt.to_vector())
vec = Lmod.coordinate_vector(elt.to_vector())
for key, coeff in vec.iteritems():
if not coeff:
continue
s, A = sgn(key, Z)
if A is None:
continue
if sparse:
coords = (row, indices[A])
if coords in data:
data[coords] += s * coeff
coords = (row, ext_ind[A])
if coords in p2_data:
p2_data[coords] += s * coeff
else:
data[coords] = s * coeff
p2_data[coords] = s * coeff
else:
ret[ext_ind[A]] += s * coeff
if sparse:
row += 1
else:
p2_data.append(ret)

nrows = binomial(len(LI), k)
ncols = binomial(len(LI), k-1)
MS = MatrixSpace(R, nrows, ncols, sparse=sparse)
if M is None:
p2 = MS(p2_data).transpose()
p2.set_immutable()
return p2
p2 = matrix.identity(len(MI)).tensor_product(MS(p2_data)).transpose()

ten_ind = {tuple(Y): i for i, Y in enumerate(product(MI, ext_ind))}

# Now compute the part from the module ("part 1")
if sparse:
p1_data = {}
row = 0
else:
p1_data = []
if not sparse:
zv = [zero] * len(ten_ind)
for v, X in product(MI, combinations(LI, k)):
if not sparse:
ret = list(zv)
for i in range(k):
# We do mone**i because we are 0-based
elt = mone**i * LB[X[i]] * MB[v]
if not elt:
continue
Y = X[:i] + X[i+1:]
for j in MI:
coeff = elt[MK[j]]
if not coeff:
continue
if sparse:
coords = (row, ten_ind[j, Y])
if coords in p1_data:
p1_data[coords] += coeff
else:
ret[indices[A]] += s * coeff
p1_data[coords] = coeff
else:
ret[ten_ind[j, Y]] += coeff
if sparse:
row += 1
else:
data.append(ret)
nrows = binomial(len(Ind), k)
ncols = binomial(len(Ind), k-1)
p1_data.append(ret)

nrows = len(MI) * binomial(len(LI), k)
ncols = len(ten_ind)
MS = MatrixSpace(R, nrows, ncols, sparse=sparse)
ret = MS(data).transpose()
ret = MS(p1_data).transpose() + p2
ret.set_immutable()
return ret

chain_data = {X[0][0]: M for X, M in compute_diff(list( range(1,len(Ind)+1) ))}

from sage.homology.chain_complex import ChainComplex
try:
return ChainComplex(chain_data, degree_of_differential=-1)
except TypeError:
return chain_data
ind = list(range(1, len(LI) + 1))
chain_data = {X[0][0]: M for X, M in compute_diff(ind)}
C = ChainComplex(chain_data, degree_of_differential=-1)
return C

def homology(self, deg=None, M=None, sparse=True, ncpus=None):
r"""
Expand Down

0 comments on commit 5a3f8eb

Please sign in to comment.