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This chapter describes functions for solving linear systems. The
library provides simple linear algebra operations which operate directly
on the gsl_vector
and gsl_matrix
objects. These are
intended for use with "small" systems where simple algorithms are
acceptable.
Anyone interested in large systems will want to use the sophisticated
routines found in LAPACK. The Fortran version of LAPACK is
recommended as the standard package for linear algebra. It supports
blocked algorithms, specialized data representations and other
optimizations.
The functions described in this chapter are declared in the header file
`gsl_linalg.h'.
A general square matrix A has an LU decomposition into
upper and lower triangular matrices,
P A = L U
where P is a permutation matrix, L is unit lower
triangular matrix and U is upper triangular matrix. For square
matrices this decomposition can be used to convert the linear system
A x = b into a pair of triangular systems (L y = P b,
U x = y), which can be solved by forward and backsubstitution.
Note that the LU decomposition is valid for singular matrices.
 Function: int gsl_linalg_LU_decomp (gsl_matrix * A, gsl_permutation * p, int *signum)

 Function: int gsl_linalg_complex_LU_decomp (gsl_matrix_complex * A, gsl_permutation * p, int *signum)

These functions factorize the square matrix A into the LU
decomposition PA = LU. On output the diagonal and upper
triangular part of the input matrix A contain the matrix
U. The lower triangular part of the input matrix (excluding the
diagonal) contains L. The diagonal elements of L are
unity, and are not stored.
The permutation matrix P is encoded in the permutation
p. The jth column of the matrix P is given by the
kth column of the identity matrix, where k = p_j the
jth element of the permutation vector. The sign of the
permutation is given by signum. It has the value (1)^n,
where n is the number of interchanges in the permutation.
The algorithm used in the decomposition is Gaussian Elimination with
partial pivoting (Golub & Van Loan, Matrix Computations,
Algorithm 3.4.1).
 Function: int gsl_linalg_LU_solve (const gsl_matrix * LU, const gsl_permutation * p, const gsl_vector * b, gsl_vector * x)

 Function: int gsl_linalg_complex_LU_solve (const gsl_matrix_complex * LU, const gsl_permutation * p, const gsl_vector_complex * b, gsl_vector_complex * x)

These functions solve the system A x = b using the LU
decomposition of A into (LU, p) given by
gsl_linalg_LU_decomp
or gsl_linalg_complex_LU_decomp
.
 Function: int gsl_linalg_LU_svx (const gsl_matrix * LU, const gsl_permutation * p, gsl_vector * x)

 Function: int gsl_linalg_complex_LU_svx (const gsl_matrix_complex * LU, const gsl_permutation * p, gsl_vector_complex * x)

These functions solve the system A x = b inplace using the
LU decomposition of A into (LU,p). On input
x should contain the righthand side b, which is replaced
by the solution on output.
 Function: int gsl_linalg_LU_refine (const gsl_matrix * A, const gsl_matrix * LU, const gsl_permutation * p, const gsl_vector * b, gsl_vector * x, gsl_vector * residual)

 Function: int gsl_linalg_complex_LU_refine (const gsl_matrix_complex * A, const gsl_matrix_complex * LU, const gsl_permutation * p, const gsl_vector_complex * b, gsl_vector_complex * x, gsl_vector_complex * residual)

These functions apply an iterative improvement to x, the solution
of A x = b, using the LU decomposition of A into
(LU,p). The initial residual r = A x  b is also
computed and stored in residual.
 Function: int gsl_linalg_LU_invert (const gsl_matrix * LU, const gsl_permutation * p, gsl_matrix * inverse)

 Function: int gsl_complex_linalg_LU_invert (const gsl_matrix_complex * LU, const gsl_permutation * p, gsl_matrix_complex * inverse)

These functions compute the inverse of a matrix A from its
LU decomposition (LU,p), storing the result in the
matrix inverse. The inverse is computed by solving the system
A x = b for each column of the identity matrix. It is preferable
to avoid direct computation of the inverse whenever possible.
 Function: double gsl_linalg_LU_det (gsl_matrix * LU, int signum)

 Function: gsl_complex gsl_linalg_complex_LU_det (gsl_matrix_complex * LU, int signum)

These functions compute the determinant of a matrix A from its
LU decomposition, LU. The determinant is computed as the
product of the diagonal elements of U and the sign of the row
permutation signum.
 Function: double gsl_linalg_LU_lndet (gsl_matrix * LU)

 Function: double gsl_linalg_complex_LU_lndet (gsl_matrix_complex * LU)

These functions compute the logarithm of the absolute value of the
determinant of a matrix A, \lndet(A), from its LU
decomposition, LU. This function may be useful if the direct
computation of the determinant would overflow or underflow.
 Function: int gsl_linalg_LU_sgndet (gsl_matrix * LU, int signum)

 Function: gsl_complex gsl_linalg_complex_LU_sgndet (gsl_matrix_complex * LU, int signum)

These functions compute the sign or phase factor of the determinant of a
matrix A, det(A)/det(A), from its LU decomposition,
LU.
A general rectangular MbyN matrix A has a
QR decomposition into the product of an orthogonal
MbyM square matrix Q (where Q^T Q = I) and
an MbyN righttriangular matrix R,
A = Q R
This decomposition can be used to convert the linear system A x =
b into the triangular system R x = Q^T b, which can be solved by
backsubstitution. Another use of the QR decomposition is to
compute an orthonormal basis for a set of vectors. The first N
columns of Q form an orthonormal basis for the range of A,
ran(A), when A has full column rank.
 Function: int gsl_linalg_QR_decomp (gsl_matrix * A, gsl_vector * tau)

This function factorizes the MbyN matrix A into
the QR decomposition A = Q R. On output the diagonal and
upper triangular part of the input matrix contain the matrix
R. The vector tau and the columns of the lower triangular
part of the matrix A contain the Householder coefficients and
Householder vectors which encode the orthogonal matrix Q. The
vector tau must be of length k=\min(M,N). The matrix
Q is related to these components by, Q = Q_k ... Q_2 Q_1
where Q_i = I  \tau_i v_i v_i^T and v_i is the
Householder vector v_i =
(0,...,1,A(i+1,i),A(i+2,i),...,A(m,i)). This is the same storage scheme
as used by LAPACK.
The algorithm used to perform the decomposition is Householder QR (Golub
& Van Loan, Matrix Computations, Algorithm 5.2.1).
 Function: int gsl_linalg_QR_solve (const gsl_matrix * QR, const gsl_vector * tau, const gsl_vector * b, gsl_vector * x)

This function solves the system A x = b using the QR
decomposition of A into (QR, tau) given by
gsl_linalg_QR_decomp
.
 Function: int gsl_linalg_QR_svx (const gsl_matrix * QR, const gsl_vector * tau, gsl_vector * x)

This function solves the system A x = b inplace using the
QR decomposition of A into (QR,tau) given by
gsl_linalg_QR_decomp
. On input x should contain the
righthand side b, which is replaced by the solution on output.
 Function: int gsl_linalg_QR_lssolve (const gsl_matrix * QR, const gsl_vector * tau, const gsl_vector * b, gsl_vector * x, gsl_vector * residual)

This function finds the least squares solution to the overdetermined
system A x = b where the matrix A has more rows than
columns. The least squares solution minimizes the Euclidean norm of the
residual, Ax  b.The routine uses the QR decomposition
of A into (QR, tau) given by
gsl_linalg_QR_decomp
. The solution is returned in x. The
residual is computed as a byproduct and stored in residual.
 Function: int gsl_linalg_QR_QTvec (const gsl_matrix * QR, const gsl_vector * tau, gsl_vector * v)

This function applies the matrix Q^T encoded in the decomposition
(QR,tau) to the vector v, storing the result Q^T
v in v. The matrix multiplication is carried out directly using
the encoding of the Householder vectors without needing to form the full
matrix Q^T.
 Function: int gsl_linalg_QR_Qvec (const gsl_matrix * QR, const gsl_vector * tau, gsl_vector * v)

This function applies the matrix Q encoded in the decomposition
(QR,tau) to the vector v, storing the result Q
v in v. The matrix multiplication is carried out directly using
the encoding of the Householder vectors without needing to form the full
matrix Q.
 Function: int gsl_linalg_QR_Rsolve (const gsl_matrix * QR, const gsl_vector * b, gsl_vector * x)

This function solves the triangular system R x = b for
x. It may be useful if the product b' = Q^T b has already
been computed using
gsl_linalg_QR_QTvec
.
 Function: int gsl_linalg_QR_Rsvx (const gsl_matrix * QR, gsl_vector * x)

This function solves the triangular system R x = b for x
inplace. On input x should contain the righthand side b
and is replaced by the solution on output. This function may be useful if
the product b' = Q^T b has already been computed using
gsl_linalg_QR_QTvec
.
 Function: int gsl_linalg_QR_unpack (const gsl_matrix * QR, const gsl_vector * tau, gsl_matrix * Q, gsl_matrix * R)

This function unpacks the encoded QR decomposition
(QR,tau) into the matrices Q and R, where
Q is MbyM and R is MbyN.
 Function: int gsl_linalg_QR_QRsolve (gsl_matrix * Q, gsl_matrix * R, const gsl_vector * b, gsl_vector * x)

This function solves the system R x = Q^T b for x. It can
be used when the QR decomposition of a matrix is available in
unpacked form as (Q,R).
 Function: int gsl_linalg_QR_update (gsl_matrix * Q, gsl_matrix * R, gsl_vector * w, const gsl_vector * v)

This function performs a rank1 update w v^T of the QR
decomposition (Q, R). The update is given by Q'R' = Q
R + w v^T where the output matrices Q' and R' are also
orthogonal and right triangular. Note that w is destroyed by the
update.
 Function: int gsl_linalg_R_solve (const gsl_matrix * R, const gsl_vector * b, gsl_vector * x)

This function solves the triangular system R x = b for the
NbyN matrix R.
 Function: int gsl_linalg_R_svx (const gsl_matrix * R, gsl_vector * x)

This function solves the triangular system R x = b inplace. On
input x should contain the righthand side b, which is
replaced by the solution on output.
The QR decomposition can be extended to the rank deficient case
by introducing a column permutation P,
A P = Q R
The first r columns of this Q form an orthonormal basis
for the range of A for a matrix with column rank r. This
decomposition can also be used to convert the linear system A x =
b into the triangular system R y = Q^T b, x = P y, which can be
solved by backsubstitution and permutation. We denote the QR
decomposition with column pivoting by QRP^T since A = Q R
P^T.
 Function: int gsl_linalg_QRPT_decomp (gsl_matrix * A, gsl_vector * tau, gsl_permutation * p, int *signum, gsl_vector * norm)

This function factorizes the MbyN matrix A into
the QRP^T decomposition A = Q R P^T. On output the
diagonal and upper triangular part of the input matrix contain the
matrix R. The permutation matrix P is stored in the
permutation p. The sign of the permutation is given by
signum. It has the value (1)^n, where n is the
number of interchanges in the permutation. The vector tau and the
columns of the lower triangular part of the matrix A contain the
Householder coefficients and vectors which encode the orthogonal matrix
Q. The vector tau must be of length k=\min(M,N). The
matrix Q is related to these components by, Q = Q_k ... Q_2
Q_1 where Q_i = I  \tau_i v_i v_i^T and v_i is the
Householder vector v_i =
(0,...,1,A(i+1,i),A(i+2,i),...,A(m,i)). This is the same storage scheme
as used by LAPACK. The vector norm is workspace of length
N used for column pivoting.
The algorithm used to perform the decomposition is Householder QR with
column pivoting (Golub & Van Loan, Matrix Computations, Algorithm
5.4.1).
 Function: int gsl_linalg_QRPT_decomp2 (const gsl_matrix * A, gsl_matrix * q, gsl_matrix * r, gsl_vector * tau, gsl_permutation * p, int *signum, gsl_vector * norm)

This function factorizes the matrix A into the decomposition
A = Q R P^T without modifying A itself and storing the
output in the separate matrices q and r.
 Function: int gsl_linalg_QRPT_solve (const gsl_matrix * QR, const gsl_vector * tau, const gsl_permutation * p, const gsl_vector * b, gsl_vector * x)

This function solves the system A x = b using the QRP^T
decomposition of A into (QR, tau, p) given by
gsl_linalg_QRPT_decomp
.
 Function: int gsl_linalg_QRPT_svx (const gsl_matrix * QR, const gsl_vector * tau, const gsl_permutation * p, gsl_vector * x)

This function solves the system A x = b inplace using the
QRP^T decomposition of A into
(QR,tau,p). On input x should contain the
righthand side b, which is replaced by the solution on output.
 Function: int gsl_linalg_QRPT_QRsolve (const gsl_matrix * Q, const gsl_matrix * R, const gsl_permutation * p, const gsl_vector * b, gsl_vector * x)

This function solves the system R P^T x = Q^T b for x. It can
be used when the QR decomposition of a matrix is available in
unpacked form as (Q,R).
 Function: int gsl_linalg_QRPT_update (gsl_matrix * Q, gsl_matrix * R, const gsl_permutation * p, gsl_vector * u, const gsl_vector * v)

This function performs a rank1 update w v^T of the QRP^T
decomposition (Q, R,p). The update is given by
Q'R' = Q R + w v^T where the output matrices Q' and
R' are also orthogonal and right triangular. Note that w is
destroyed by the update. The permutation p is not changed.
 Function: int gsl_linalg_QRPT_Rsolve (const gsl_matrix * QR, const gsl_permutation * p, const gsl_vector * b, gsl_vector * x)

This function solves the triangular system R P^T x = b for the
NbyN matrix R contained in QR.
 Function: int gsl_linalg_QRPT_Rsvx (const gsl_matrix * QR, const gsl_permutation * p, gsl_vector * x)

This function solves the triangular system R P^T x = b inplace
for the NbyN matrix R contained in QR. On
input x should contain the righthand side b, which is
replaced by the solution on output.
A general rectangular MbyN matrix A has a
singular value decomposition (SVD) into the product of an
MbyN orthogonal matrix U, an NbyN
diagonal matrix of singular values S and the transpose of an
NbyN orthogonal square matrix V,
A = U S V^T
The singular values
\sigma_i = S_{ii} are all nonnegative and are
generally chosen to form a nonincreasing sequence
\sigma_1 >= \sigma_2 >= ... >= \sigma_N >= 0.
The singular value decomposition of a matrix has many practical uses.
The condition number of the matrix is given by the ratio of the largest
singular value to the smallest singular value. The presence of a zero
singular value indicates that the matrix is singular. The number of
nonzero singular values indicates the rank of the matrix. In practice
singular value decomposition of a rankdeficient matrix will not produce
exact zeroes for singular values, due to finite numerical
precision. Small singular values should be edited by choosing a suitable
tolerance.
 Function: int gsl_linalg_SV_decomp (gsl_matrix * A, gsl_matrix * V, gsl_vector * S, gsl_vector * work)

This function factorizes the MbyN matrix A into
the singular value decomposition A = U S V^T. On output the
matrix A is replaced by U. The diagonal elements of the
singular value matrix S are stored in the vector S. The
singular values are nonnegative and form a nonincreasing sequence from
S_1 to S_N. The matrix V contains the elements of
V in untransposed form. To form the product U S V^T it is
necessary to take the transpose of V. A workspace of length
N is required in work.
This routine uses the GolubReinsch SVD algorithm.
 Function: int gsl_linalg_SV_decomp_mod (gsl_matrix * A, gsl_matrix * X, gsl_matrix * V, gsl_vector * S, gsl_vector * work)

This function computes the SVD using the modified GolubReinsch
algorithm, which is faster for
M>>N. It requires the vector
work and the NbyN matrix X as additional
working space.
 Function: int gsl_linalg_SV_decomp_jacobi (gsl_matrix * A, gsl_matrix * V, gsl_vector * S)

This function computes the SVD using onesided Jacobi orthogonalization
(see references for details). The Jacobi method can compute singular
values to higher relative accuracy than GolubReinsch algorithms.
 Function: int gsl_linalg_SV_solve (gsl_matrix * U, gsl_matrix * V, gsl_vector * S, const gsl_vector * b, gsl_vector * x)

This function solves the system A x = b using the singular value
decomposition (U, S, V) of A given by
gsl_linalg_SV_decomp
.
Only nonzero singular values are used in computing the solution. The
parts of the solution corresponding to singular values of zero are
ignored. Other singular values can be edited out by setting them to
zero before calling this function.
In the overdetermined case where A has more rows than columns the
system is solved in the least squares sense, returning the solution
x which minimizes A x  b_2.
A symmetric, positive definite square matrix A has a Cholesky
decomposition into a product of a lower triangular matrix L and
its transpose L^T,
A = L L^T
This is sometimes referred to as taking the squareroot of a matrix. The
Cholesky decomposition can only be carried out when all the eigenvalues
of the matrix are positive. This decomposition can be used to convert
the linear system A x = b into a pair of triangular systems
(L y = b, L^T x = y), which can be solved by forward and
backsubstitution.
 Function: int gsl_linalg_cholesky_decomp (gsl_matrix * A)

This function factorizes the positivedefinite square matrix A
into the Cholesky decomposition A = L L^T. On output the diagonal
and lower triangular part of the input matrix A contain the matrix
L. The upper triangular part of the input matrix contains
L^T, the diagonal terms being identical for both L and
L^T. If the matrix is not positivedefinite then the
decomposition will fail, returning the error code
GSL_EDOM
.
 Function: int gsl_linalg_cholesky_solve (const gsl_matrix * cholesky, const gsl_vector * b, gsl_vector * x)

This function solves the system A x = b using the Cholesky
decomposition of A into the matrix cholesky given by
gsl_linalg_cholesky_decomp
.
 Function: int gsl_linalg_cholesky_svx (const gsl_matrix * cholesky, gsl_vector * x)

This function solves the system A x = b inplace using the
Cholesky decomposition of A into the matrix cholesky given
by
gsl_linalg_cholesky_decomp
. On input x should contain
the righthand side b, which is replaced by the solution on
output.
A symmetric matrix A can be factorized by similarity
transformations into the form,
A = Q T Q^T
where Q is an orthogonal matrix and T is a symmetric
tridiagonal matrix.
 Function: int gsl_linalg_symmtd_decomp (gsl_matrix * A, gsl_vector * tau)

This function factorizes the symmetric square matrix A into the
symmetric tridiagonal decomposition Q T Q^T. On output the
diagonal and subdiagonal part of the input matrix A contain the
tridiagonal matrix T. The remaining lower triangular part of the
input matrix contains the Householder vectors which, together with the
Householder coefficients tau, encode the orthogonal matrix
Q. This storage scheme is the same as used by LAPACK. The
upper triangular part of A is not referenced.
 Function: int gsl_linalg_symmtd_unpack (const gsl_matrix * A, const gsl_vector * tau, gsl_matrix * Q, gsl_vector * diag, gsl_vector * subdiag)

This function unpacks the encoded symmetric tridiagonal decomposition
(A, tau) obtained from
gsl_linalg_symmtd_decomp
into
the orthogonal matrix Q, the vector of diagonal elements diag
and the vector of subdiagonal elements subdiag.
 Function: int gsl_linalg_symmtd_unpack_T (const gsl_matrix * A, gsl_vector * diag, gsl_vector * subdiag)

This function unpacks the diagonal and subdiagonal of the encoded
symmetric tridiagonal decomposition (A, tau) obtained from
gsl_linalg_symmtd_decomp
into the vectors diag and subdiag.
A hermitian matrix A can be factorized by similarity
transformations into the form,
A = U T U^T
where U is an unitary matrix and T is a real symmetric
tridiagonal matrix.
 Function: int gsl_linalg_hermtd_decomp (gsl_matrix_complex * A, gsl_vector_complex * tau)

This function factorizes the hermitian matrix A into the symmetric
tridiagonal decomposition U T U^T. On output the real parts of
the diagonal and subdiagonal part of the input matrix A contain
the tridiagonal matrix T. The remaining lower triangular part of
the input matrix contains the Householder vectors which, together with
the Householder coefficients tau, encode the orthogonal matrix
Q. This storage scheme is the same as used by LAPACK. The
upper triangular part of A and imaginary parts of the diagonal are
not referenced.
 Function: int gsl_linalg_hermtd_unpack (const gsl_matrix_complex * A, const gsl_vector_complex * tau, gsl_matrix_complex * Q, gsl_vector * diag, gsl_vector * subdiag)

This function unpacks the encoded tridiagonal decomposition (A,
tau) obtained from
gsl_linalg_hermtd_decomp
into the
unitary matrix U, the real vector of diagonal elements diag and
the real vector of subdiagonal elements subdiag.
 Function: int gsl_linalg_hermtd_unpack_T (const gsl_matrix_complex * A, gsl_vector * diag, gsl_vector * subdiag)

This function unpacks the diagonal and subdiagonal of the encoded
tridiagonal decomposition (A, tau) obtained from
gsl_linalg_hermtd_decomp
into the real vectors diag and
subdiag.
A general matrix A can be factorized by similarity
transformations into the form,
A = U B V^T
where U and V are orthogonal matrices and B is a
NbyN bidiagonal matrix with nonzero entries only on the
diagonal and superdiagonal. The size of U is MbyN
and the size of V is NbyN.
 Function: int gsl_linalg_bidiag_decomp (gsl_matrix * A, gsl_vector * tau_U, gsl_vector * tau_V)

This function factorizes the MbyN matrix A into
bidiagonal form U B V^T. The diagonal and superdiagonal of the
matrix B are stored in the diagonal and superdiagonal of A.
The orthogonal matrices U and V are stored as compressed
Householder vectors in the remaining elements of A. The
Householder coefficients are stored in the vectors tau_U and
tau_V. The length of tau_U must equal the number of
elements in the diagonal of A and the length of tau_V should
be one element shorter.
 Function: int gsl_linalg_bidiag_unpack (const gsl_matrix * A, const gsl_vector * tau_U, gsl_matrix * U, const gsl_vector * tau_V, gsl_matrix * V, gsl_vector * diag, gsl_vector * superdiag)

This function unpacks the bidiagonal decomposition of A given by
gsl_linalg_bidiag_decomp
, (A, tau_U, tau_V)
into the separate orthogonal matrices U, V and the diagonal
vector diag and superdiagonal superdiag. Note that U
is stored as a compact MbyN orthogonal matrix satisfying
U^T U = I for efficiency.
 Function: int gsl_linalg_bidiag_unpack2 (gsl_matrix * A, gsl_vector * tau_U, gsl_vector * tau_V, gsl_matrix * V)

This function unpacks the bidiagonal decomposition of A given by
gsl_linalg_bidiag_decomp
, (A, tau_U, tau_V)
into the separate orthogonal matrices U, V and the diagonal
vector diag and superdiagonal superdiag. The matrix U
is stored inplace in A.
 Function: int gsl_linalg_bidiag_unpack_B (const gsl_matrix * A, gsl_vector * diag, gsl_vector * superdiag)

This function unpacks the diagonal and superdiagonal of the bidiagonal
decomposition of A given by
gsl_linalg_bidiag_decomp
, into
the diagonal vector diag and superdiagonal vector superdiag.
A Householder transformation is a rank1 modification of the identity
matrix which can be used to zero out selected elements of a vector. A
Householder matrix P takes the form,
P = I  \tau v v^T
where v is a vector (called the Householder vector) and
\tau = 2/(v^T v). The functions described in this section use the
rank1 structure of the Householder matrix to create and apply
Householder transformations efficiently.
 Function: double gsl_linalg_houlsehoulder_transform (gsl_vector * v)

This function prepares a Householder transformation P = I  \tau v
v^T which can be used to zero all the elements of the input vector except
the first. On output the transformation is stored in the vector v
and the scalar \tau is returned.
 Function: int gsl_linalg_householder_hm (double tau, const gsl_vector * v, gsl_matrix * A)

This function applies the Householder matrix P defined by the
scalar tau and the vector v to the lefthand side of the
matrix A. On output the result P A is stored in A.
 Function: int gsl_linalg_householder_mh (double tau, const gsl_vector * v, gsl_matrix * A)

This function applies the Householder matrix P defined by the
scalar tau and the vector v to the righthand side of the
matrix A. On output the result A P is stored in A.
 Function: int gsl_linalg_householder_hv (double tau, const gsl_vector * v, gsl_vector * w)

This function applies the Householder transformation P defined by
the scalar tau and the vector v to the vector w. On
output the result P w is stored in w.
 Function: int gsl_linalg_HH_solve (gsl_matrix * A, const gsl_vector * b, gsl_vector * x)

This function solves the system A x = b directly using
Householder transformations. On output the solution is stored in x
and b is not modified. The matrix A is destroyed by the
Householder transformations.
 Function: int gsl_linalg_HH_svx (gsl_matrix * A, gsl_vector * x)

This function solves the system A x = b inplace using
Householder transformations. On input x should contain the
righthand side b, which is replaced by the solution on output. The
matrix A is destroyed by the Householder transformations.
 Function: int gsl_linalg_solve_symm_tridiag (const gsl_vector * diag, const gsl_vector * e, const gsl_vector * b, gsl_vector * x)

This function solves the general NbyN system A x =
b where A is symmetric tridiagonal. The form of A for the
4by4 case is shown below,
A = ( d_0 e_0 )
( e_0 d_1 e_1 )
( e_1 d_2 e_2 )
( e_2 d_3 )
 Function: int gsl_linalg_solve_symm_cyc_tridiag (const gsl_vector * diag, const gsl_vector * e, const gsl_vector * b, gsl_vector * x)

This function solves the general NbyN system A x =
b where A is symmetric cyclic tridiagonal. The form of A
for the 4by4 case is shown below,
A = ( d_0 e_0 e_3 )
( e_0 d_1 e_1 )
( e_1 d_2 e_2 )
( e_3 e_2 d_3 )
The following program solves the linear system A x = b. The
system to be solved is,
[ 0.18 0.60 0.57 0.96 ] [x0] [1.0]
[ 0.41 0.24 0.99 0.58 ] [x1] = [2.0]
[ 0.14 0.30 0.97 0.66 ] [x2] [3.0]
[ 0.51 0.13 0.19 0.85 ] [x3] [4.0]
and the solution is found using LU decomposition of the matrix A.
#include <stdio.h>
#include <gsl/gsl_linalg.h>
int
main (void)
{
double a_data[] = { 0.18, 0.60, 0.57, 0.96,
0.41, 0.24, 0.99, 0.58,
0.14, 0.30, 0.97, 0.66,
0.51, 0.13, 0.19, 0.85 };
double b_data[] = { 1.0, 2.0, 3.0, 4.0 };
gsl_matrix_view m
= gsl_matrix_view_array (a_data, 4, 4);
gsl_vector_view b
= gsl_vector_view_array (b_data, 4);
gsl_vector *x = gsl_vector_alloc (4);
int s;
gsl_permutation * p = gsl_permutation_alloc (4);
gsl_linalg_LU_decomp (&m.matrix, p, &s);
gsl_linalg_LU_solve (&m.matrix, p, &b.vector, x);
printf ("x = \n");
gsl_vector_fprintf (stdout, x, "%g");
gsl_permutation_free (p);
return 0;
}
Here is the output from the program,
x = 4.05205
12.6056
1.66091
8.69377
This can be verified by multiplying the solution x by the
original matrix A using GNU OCTAVE,
octave> A = [ 0.18, 0.60, 0.57, 0.96;
0.41, 0.24, 0.99, 0.58;
0.14, 0.30, 0.97, 0.66;
0.51, 0.13, 0.19, 0.85 ];
octave> x = [ 4.05205; 12.6056; 1.66091; 8.69377];
octave> A * x
ans =
1.0000
2.0000
3.0000
4.0000
This reproduces the original righthand side vector, b, in
accordance with the equation A x = b.
Further information on the algorithms described in this section can be
found in the following book,

G. H. Golub, C. F. Van Loan, Matrix Computations (3rd Ed, 1996),
Johns Hopkins University Press, ISBN 0801854148.
The LAPACK library is described in the following manual,
The LAPACK source code can be found at the website above, along
with an online copy of the users guide.
The Modified GolubReinsch algorithm is described in the following paper,

T.F. Chan, "An Improved Algorithm for Computing the Singular Value
Decomposition", ACM Transactions on Mathematical Software, 8
(1982), pp 7283.
The Jacobi algorithm for singular value decomposition is described in
the following papers,

J.C.Nash, "A onesided transformation method for the singular value
decomposition and algebraic eigenproblem", Computer Journal,
Volume 18, Number 1 (1973), p 7476

James Demmel, Kresimir Veselic, "Jacobi's Method is more accurate than
QR", Lapack Working Note 15 (LAWN15), October 1989. Available
from netlib, http://www.netlib.org/lapack/ in the
lawns
or
lawnspdf
directories.
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