Sparse Linear Equations
In this section we describe routines for solving sparse sets of linear equations.
A real symmetric or complex Hermitian sparse matrix is stored as an
spmatrix
object X
of size
(, ) and an
additional character argument uplo
with possible values 'L'
and 'U'
. If uplo
is 'L'
, the lower triangular part
of X
contains the lower triangular part of the symmetric or Hermitian
matrix, and the upper triangular matrix of X
is ignored. If uplo
is 'U'
, the upper triangular part of X
contains the upper
triangular part of the matrix, and the lower triangular matrix of X
is
ignored.
A general sparse square matrix of order is represented by an
spmatrix
object of size (, ).
Dense matrices, which appear as right-hand sides of equations, are stored using the same conventions as in the BLAS and LAPACK modules.
Matrix Orderings
CVXOPT includes an interface to the AMD library for computing approximate minimum degree orderings of sparse matrices.
See also
P. R. Amestoy, T. A. Davis, I. S. Duff, Algorithm 837: AMD, An Approximate Minimum Degree Ordering Algorithm, ACM Transactions on Mathematical Software, 30(3), 381-388, 2004.
- cvxopt.amd.order(A[, uplo = 'L'])
Computes the approximate mimimum degree ordering of a symmetric sparse matrix . The ordering is returned as an integer dense matrix with length equal to the order of . Its entries specify a permutation that reduces fill-in during the Cholesky factorization. More precisely, if
p = order(A)
, thenA[p, p]
has sparser Cholesky factors thanA
.
As an example we consider the matrix
>>> from cvxopt import spmatrix, amd
>>> A = spmatrix([10,3,5,-2,5,2], [0,2,1,2,2,3], [0,0,1,1,2,3])
>>> P = amd.order(A)
>>> print(P)
[ 1]
[ 0]
[ 2]
[ 3]
General Linear Equations
The module cvxopt.umfpack
includes four functions for solving
sparse non-symmetric sets of linear equations. They call routines from
the UMFPACK library, with all control options set to the default values
described in the UMFPACK user guide.
See also
T. A. Davis, Algorithm 832: UMFPACK – an unsymmetric-pattern multifrontal method with a column pre-ordering strategy, ACM Transactions on Mathematical Software, 30(2), 196-199, 2004.
- cvxopt.umfpack.linsolve(A, B[, trans = 'N'])
Solves a sparse set of linear equations
where is a sparse matrix and is a dense matrix. The arguments
A
andB
must have the same type ('d'
or'z'
) asA
. On exitB
contains the solution. Raises anArithmeticError
if the coefficient matrix is singular.
In the following example we solve an equation with coefficient matrix
(1)
>>> from cvxopt import spmatrix, matrix, umfpack
>>> V = [2, 3, 3, -1, 4, 4, -3, 1, 2, 2, 6, 1]
>>> I = [0, 1, 0, 2, 4, 1, 2, 3, 4, 2, 1, 4]
>>> J = [0, 0, 1, 1, 1, 2, 2, 2, 2, 3, 4, 4]
>>> A = spmatrix(V,I,J)
>>> B = matrix(1.0, (5,1))
>>> umfpack.linsolve(A,B)
>>> print(B)
[ 5.79e-01]
[-5.26e-02]
[ 1.00e+00]
[ 1.97e+00]
[-7.89e-01]
The function linsolve
is
equivalent to the following three functions called in sequence.
- cvxopt.umfpack.symbolic(A)
Reorders the columns of
A
to reduce fill-in and performs a symbolic LU factorization.A
is a sparse, possibly rectangular, matrix. Returns the symbolic factorization as an opaque C object that can be passed on tonumeric
.
- cvxopt.umfpack.numeric(A, F)
Performs a numeric LU factorization of a sparse, possibly rectangular, matrix
A
. The argumentF
is the symbolic factorization computed bysymbolic
applied to the matrixA
, or another sparse matrix with the same sparsity pattern, dimensions, and type. The numeric factorization is returned as an opaque C object that that can be passed on tosolve
. Raises anArithmeticError
if the matrix is singular.
- cvxopt.umfpack.solve(A, F, B[, trans = 'N'])
Solves a set of linear equations
where is a sparse matrix and is a dense matrix. The arguments
A
andB
must have the same type. The argumentF
is a numeric factorization computed bynumeric
. On exitB
is overwritten by the solution.
These separate functions are useful for solving several sets of linear equations with the same coefficient matrix and different right-hand sides, or with coefficient matrices that share the same sparsity pattern. The symbolic factorization depends only on the sparsity pattern of the matrix, and not on the numerical values of the nonzero coefficients. The numerical factorization on the other hand depends on the sparsity pattern of the matrix and on its the numerical values.
As an example, suppose is the matrix (1) and
which differs from in its first and last entries. The following code computes
>>> from cvxopt import spmatrix, matrix, umfpack
>>> VA = [2, 3, 3, -1, 4, 4, -3, 1, 2, 2, 6, 1]
>>> VB = [4, 3, 3, -1, 4, 4, -3, 1, 2, 2, 6, 2]
>>> I = [0, 1, 0, 2, 4, 1, 2, 3, 4, 2, 1, 4]
>>> J = [0, 0, 1, 1, 1, 2, 2, 2, 2, 3, 4, 4]
>>> A = spmatrix(VA, I, J)
>>> B = spmatrix(VB, I, J)
>>> x = matrix(1.0, (5,1))
>>> Fs = umfpack.symbolic(A)
>>> FA = umfpack.numeric(A, Fs)
>>> FB = umfpack.numeric(B, Fs)
>>> umfpack.solve(A, FA, x)
>>> umfpack.solve(B, FB, x)
>>> umfpack.solve(A, FA, x, trans='T')
>>> print(x)
[ 5.81e-01]
[-2.37e-01]
[ 1.63e+00]
[ 8.07e+00]
[-1.31e-01]
Positive Definite Linear Equations
cvxopt.cholmod
is an interface to the Cholesky factorization routines
of the CHOLMOD package. It includes functions for Cholesky factorization
of sparse positive definite matrices, and for solving sparse sets of linear
equations with positive definite matrices.
The routines can also be used for computing
LDL^{T}
(or
LDL^{H}
factorizations
of symmetric indefinite matrices (with unit lower-triangular and
diagonal and nonsingular) if such a factorization exists.
See also
Y. Chen, T. A. Davis, W. W. Hager, S. Rajamanickam, Algorithm 887: CHOLMOD, Supernodal Sparse Cholesky Factorization and Update/Downdate, ACM Transactions on Mathematical Software, 35(3), 22:1-22:14, 2008.
- cvxopt.cholmod.linsolve(A, B[, p = None, uplo = 'L'])
Solves
with sparse and real symmetric or complex Hermitian.
B
is a dense matrix of the same type asA
. On exit it is overwritten with the solution. The argumentp
is an integer matrix with length equal to the order of , and specifies an optional reordering. See the comment onoptions['nmethods']
for details on which ordering is used by CHOLMOD.Raises an
ArithmeticError
if the factorization does not exist.
As an example, we solve
(2)
>>> from cvxopt import matrix, spmatrix, cholmod
>>> A = spmatrix([10, 3, 5, -2, 5, 2], [0, 2, 1, 3, 2, 3], [0, 0, 1, 1, 2, 3])
>>> X = matrix(range(8), (4,2), 'd')
>>> cholmod.linsolve(A,X)
>>> print(X)
[-1.46e-01 4.88e-02]
[ 1.33e+00 4.00e+00]
[ 4.88e-01 1.17e+00]
[ 2.83e+00 7.50e+00]
- cvxopt.cholmod.splinsolve(A, B[, p = None, uplo = 'L'])
Similar to
linsolve
except thatB
is anspmatrix
and that the solution is returned as an output argument (as a newspmatrix
).B
is not modified. See the comment onoptions['nmethods']
for details on which ordering is used by CHOLMOD.
The following code computes the inverse of the coefficient matrix in (2) as a sparse matrix.
>>> X = cholmod.splinsolve(A, spmatrix(1.0,range(4),range(4)))
>>> print(X)
[ 1.22e-01 0 -7.32e-02 0 ]
[ 0 3.33e-01 0 3.33e-01]
[-7.32e-02 0 2.44e-01 0 ]
[ 0 3.33e-01 0 8.33e-01]
The functions linsolve
and
splinsolve
are equivalent to
symbolic
and
numeric
called in sequence, followed by
solve
, respectively,
spsolve
.
- cvxopt.cholmod.symbolic(A[, p = None, uplo = 'L'])
Performs a symbolic analysis of a sparse real symmetric or complex Hermitian matrix for one of the two factorizations:
(3)
and
(4)
where is a permutation matrix, is lower triangular (unit lower triangular in the second factorization), and is nonsingular diagonal. The type of factorization depends on the value of
options['supernodal']
(see below).If
uplo
is'L'
, only the lower triangular part ofA
is accessed and the upper triangular part is ignored. Ifuplo
is'U'
, only the upper triangular part ofA
is accessed and the lower triangular part is ignored.The symbolic factorization is returned as an opaque C object that can be passed to
numeric
.See the comment on
options['nmethods']
for details on which ordering is used by CHOLMOD.
- cvxopt.cholmod.numeric(A, F)
Performs a numeric factorization of a sparse symmetric matrix as (3) or (4). The argument
F
is the symbolic factorization computed bysymbolic
applied to the matrixA
, or to another sparse matrix with the same sparsity pattern and typecode, or bynumeric
applied to a matrix with the same sparsity pattern and typecode asA
.If
F
was created by asymbolic
withuplo
equal to'L'
, then only the lower triangular part ofA
is accessed and the upper triangular part is ignored. If it was created withuplo
equal to'U'
, then only the upper triangular part ofA
is accessed and the lower triangular part is ignored.On successful exit, the factorization is stored in
F
. Raises anArithmeticError
if the factorization does not exist.
- cvxopt.cholmod.solve(F, B[, sys = 0])
Solves one of the following linear equations where
B
is a dense matrix andF
is the numeric factorization (3) or (4) computed bynumeric
.sys
is an integer with values between 0 and 8.sys
equation
0
1
2
3
4
5
6
7
8
(If
F
is a Cholesky factorization of the form (3), is an identity matrix in this table. IfA
is complex, should be replaced by .)The matrix
B
is a dense'd'
or'z'
matrix, with the same type asA
. On exit it is overwritten by the solution.
- cvxopt.cholmod.spsolve(F, B[, sys = 0])
Similar to
solve
, except thatB
is a class:spmatrix, and the solution is returned as an output argument (as anspmatrix
).B
must have the same typecode asA
.
For the same example as above:
>>> X = matrix(range(8), (4,2), 'd')
>>> F = cholmod.symbolic(A)
>>> cholmod.numeric(A, F)
>>> cholmod.solve(F, X)
>>> print(X)
[-1.46e-01 4.88e-02]
[ 1.33e+00 4.00e+00]
[ 4.88e-01 1.17e+00]
[ 2.83e+00 7.50e+00]
- cvxopt.cholmod.diag(F)
Returns the diagonal elements of the Cholesky factor in (3), as a dense matrix of the same type as
A
. Note that this only applies to Cholesky factorizations. The matrix in an LDL^{T} factorization can be retrieved viasolve
withsys
equal to 6.
In the functions listed above, the default values of the control
parameters described in the CHOLMOD user guide are used, except for
Common.print
which is set to 0 instead of 3 and
Common.supernodal
which is set to 2 instead of 1.
These parameters (and a few others) can be modified by making an
entry in the dictionary cholmod.options
.
The meaning of the options options['supernodal']
and
options['nmethods']
is summarized as follows (and described
in detail in the CHOLMOD user guide).
options['supernodal']
If equal to 0, a factorization (4) is computed using a simplicial algorithm. If equal to 2, a factorization (3) is computed using a supernodal algorithm. If equal to 1, the most efficient of the two factorizations is selected, based on the sparsity pattern. Default: 2.
options['nmethods']
The default ordering used by the CHOLMOD is the ordering in the AMD library, but depending on the value of
options['nmethods']
. other orderings are also considered. Ifnmethods
is equal to 2, the ordering specified by the user and the AMD ordering are compared, and the best of the two orderings is used. If the user does not specify an ordering, the AMD ordering is used. If equal to 1, the user must specify an ordering, and the ordering provided by the user is used. If equal to 0, all available orderings are compared and the best ordering is used. The available orderings include the AMD ordering, the ordering specified by the user (if any), and possibly other orderings if they are installed during the CHOLMOD installation. Default: 0.
As an example that illustrates diag
and the
use of cholmod.options
, we compute the logarithm of the determinant
of the coefficient matrix in (2) by two methods.
>>> import math
>>> from cvxopt.cholmod import options
>>> from cvxopt import log
>>> F = cholmod.symbolic(A)
>>> cholmod.numeric(A, F)
>>> print(2.0 * sum(log(cholmod.diag(F))))
5.50533153593
>>> options['supernodal'] = 0
>>> F = cholmod.symbolic(A)
>>> cholmod.numeric(A, F)
>>> Di = matrix(1.0, (4,1))
>>> cholmod.solve(F, Di, sys=6)
>>> print(-sum(log(Di)))
5.50533153593
Example: Covariance Selection
This example illustrates the use of the routines for sparse Cholesky factorization. We consider the problem
(5)
The optimization variable is a symmetric matrix of order and the domain of the problem is the set of positive definite matrices. The matrix and the index set are given. We assume that all the diagonal positions are included in . This problem arises in maximum likelihood estimation of the covariance matrix of a zero-mean normal distribution, with constraints that specify that pairs of variables are conditionally independent.
We can express as
where are the nonzero elements in the lower triangular part of , with the diagonal elements scaled by 1/2, and
where (, ) are the positions of the nonzero entries in the lower-triangular part of . With this notation, we can solve problem (5) by solving the unconstrained problem
The code below implements Newton’s method with a backtracking line search. The gradient and Hessian of the objective function are given by
where denotes Hadamard product.
from cvxopt import matrix, spmatrix, log, mul, blas, lapack, amd, cholmod
def covsel(Y):
"""
Returns the solution of
minimize -log det K + Tr(KY)
subject to K_{ij}=0, (i,j) not in indices listed in I,J.
Y is a symmetric sparse matrix with nonzero diagonal elements.
I = Y.I, J = Y.J.
"""
I, J = Y.I, Y.J
n, m = Y.size[0], len(I)
N = I + J*n # non-zero positions for one-argument indexing
D = [k for k in range(m) if I[k]==J[k]] # position of diagonal elements
# starting point: symmetric identity with nonzero pattern I,J
K = spmatrix(0.0, I, J)
K[::n+1] = 1.0
# Kn is used in the line search
Kn = spmatrix(0.0, I, J)
# symbolic factorization of K
F = cholmod.symbolic(K)
# Kinv will be the inverse of K
Kinv = matrix(0.0, (n,n))
for iters in range(100):
# numeric factorization of K
cholmod.numeric(K, F)
d = cholmod.diag(F)
# compute Kinv by solving K*X = I
Kinv[:] = 0.0
Kinv[::n+1] = 1.0
cholmod.solve(F, Kinv)
# solve Newton system
grad = 2*(Y.V - Kinv[N])
hess = 2*(mul(Kinv[I,J],Kinv[J,I]) + mul(Kinv[I,I],Kinv[J,J]))
v = -grad
lapack.posv(hess,v)
# stopping criterion
sqntdecr = -blas.dot(grad,v)
print("Newton decrement squared:%- 7.5e" %sqntdecr)
if (sqntdecr < 1e-12):
print("number of iterations: ", iters+1)
break
# line search
dx = +v
dx[D] *= 2 # scale the diagonal elems
f = -2.0 * sum(log(d)) # f = -log det K
s = 1
for lsiter in range(50):
Kn.V = K.V + s*dx
try:
cholmod.numeric(Kn, F)
except ArithmeticError:
s *= 0.5
else:
d = cholmod.diag(F)
fn = -2.0 * sum(log(d)) + 2*s*blas.dot(v,Y.V)
if (fn < f - 0.01*s*sqntdecr):
break
s *= 0.5
K.V = Kn.V
return K