# DOUBLE PRECISION routines for triangular band matrix

## dtbcon

```USAGE:
rcond, info = NumRu::Lapack.dtbcon( norm, uplo, diag, kd, ab, [:usage => usage, :help => help])

FORTRAN MANUAL
SUBROUTINE DTBCON( NORM, UPLO, DIAG, N, KD, AB, LDAB, RCOND, WORK, IWORK, INFO )

*  Purpose
*  =======
*
*  DTBCON estimates the reciprocal of the condition number of a
*  triangular band matrix A, in either the 1-norm or the infinity-norm.
*
*  The norm of A is computed and an estimate is obtained for
*  norm(inv(A)), then the reciprocal of the condition number is
*  computed as
*     RCOND = 1 / ( norm(A) * norm(inv(A)) ).
*

*  Arguments
*  =========
*
*  NORM    (input) CHARACTER*1
*          Specifies whether the 1-norm condition number or the
*          infinity-norm condition number is required:
*          = '1' or 'O':  1-norm;
*          = 'I':         Infinity-norm.
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  A is upper triangular;
*          = 'L':  A is lower triangular.
*
*  DIAG    (input) CHARACTER*1
*          = 'N':  A is non-unit triangular;
*          = 'U':  A is unit triangular.
*
*  N       (input) INTEGER
*          The order of the matrix A.  N >= 0.
*
*  KD      (input) INTEGER
*          The number of superdiagonals or subdiagonals of the
*          triangular band matrix A.  KD >= 0.
*
*  AB      (input) DOUBLE PRECISION array, dimension (LDAB,N)
*          The upper or lower triangular band matrix A, stored in the
*          first kd+1 rows of the array. The j-th column of A is stored
*          in the j-th column of the array AB as follows:
*          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
*          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
*          If DIAG = 'U', the diagonal elements of A are not referenced
*          and are assumed to be 1.
*
*  LDAB    (input) INTEGER
*          The leading dimension of the array AB.  LDAB >= KD+1.
*
*  RCOND   (output) DOUBLE PRECISION
*          The reciprocal of the condition number of the matrix A,
*          computed as RCOND = 1/(norm(A) * norm(inv(A))).
*
*  WORK    (workspace) DOUBLE PRECISION array, dimension (3*N)
*
*  IWORK   (workspace) INTEGER array, dimension (N)
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value
*

*  =====================================================================
*

```
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## dtbrfs

```USAGE:
ferr, berr, info = NumRu::Lapack.dtbrfs( uplo, trans, diag, kd, ab, b, x, [:usage => usage, :help => help])

FORTRAN MANUAL
SUBROUTINE DTBRFS( UPLO, TRANS, DIAG, N, KD, NRHS, AB, LDAB, B, LDB, X, LDX, FERR, BERR, WORK, IWORK, INFO )

*  Purpose
*  =======
*
*  DTBRFS provides error bounds and backward error estimates for the
*  solution to a system of linear equations with a triangular band
*  coefficient matrix.
*
*  The solution matrix X must be computed by DTBTRS or some other
*  means before entering this routine.  DTBRFS does not do iterative
*  refinement because doing so cannot improve the backward error.
*

*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  A is upper triangular;
*          = 'L':  A is lower triangular.
*
*  TRANS   (input) CHARACTER*1
*          Specifies the form of the system of equations:
*          = 'N':  A * X = B  (No transpose)
*          = 'T':  A**T * X = B  (Transpose)
*          = 'C':  A**H * X = B  (Conjugate transpose = Transpose)
*
*  DIAG    (input) CHARACTER*1
*          = 'N':  A is non-unit triangular;
*          = 'U':  A is unit triangular.
*
*  N       (input) INTEGER
*          The order of the matrix A.  N >= 0.
*
*  KD      (input) INTEGER
*          The number of superdiagonals or subdiagonals of the
*          triangular band matrix A.  KD >= 0.
*
*  NRHS    (input) INTEGER
*          The number of right hand sides, i.e., the number of columns
*          of the matrices B and X.  NRHS >= 0.
*
*  AB      (input) DOUBLE PRECISION array, dimension (LDAB,N)
*          The upper or lower triangular band matrix A, stored in the
*          first kd+1 rows of the array. The j-th column of A is stored
*          in the j-th column of the array AB as follows:
*          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
*          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
*          If DIAG = 'U', the diagonal elements of A are not referenced
*          and are assumed to be 1.
*
*  LDAB    (input) INTEGER
*          The leading dimension of the array AB.  LDAB >= KD+1.
*
*  B       (input) DOUBLE PRECISION array, dimension (LDB,NRHS)
*          The right hand side matrix B.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B.  LDB >= max(1,N).
*
*  X       (input) DOUBLE PRECISION array, dimension (LDX,NRHS)
*          The solution matrix X.
*
*  LDX     (input) INTEGER
*          The leading dimension of the array X.  LDX >= max(1,N).
*
*  FERR    (output) DOUBLE PRECISION array, dimension (NRHS)
*          The estimated forward error bound for each solution vector
*          X(j) (the j-th column of the solution matrix X).
*          If XTRUE is the true solution corresponding to X(j), FERR(j)
*          is an estimated upper bound for the magnitude of the largest
*          element in (X(j) - XTRUE) divided by the magnitude of the
*          largest element in X(j).  The estimate is as reliable as
*          the estimate for RCOND, and is almost always a slight
*          overestimate of the true error.
*
*  BERR    (output) DOUBLE PRECISION array, dimension (NRHS)
*          The componentwise relative backward error of each solution
*          vector X(j) (i.e., the smallest relative change in
*          any element of A or B that makes X(j) an exact solution).
*
*  WORK    (workspace) DOUBLE PRECISION array, dimension (3*N)
*
*  IWORK   (workspace) INTEGER array, dimension (N)
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value
*

*  =====================================================================
*

```
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## dtbtrs

```USAGE:
info, b = NumRu::Lapack.dtbtrs( uplo, trans, diag, kd, ab, b, [:usage => usage, :help => help])

FORTRAN MANUAL
SUBROUTINE DTBTRS( UPLO, TRANS, DIAG, N, KD, NRHS, AB, LDAB, B, LDB, INFO )

*  Purpose
*  =======
*
*  DTBTRS solves a triangular system of the form
*
*     A * X = B  or  A**T * X = B,
*
*  where A is a triangular band matrix of order N, and B is an
*  N-by NRHS matrix.  A check is made to verify that A is nonsingular.
*

*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  A is upper triangular;
*          = 'L':  A is lower triangular.
*
*  TRANS   (input) CHARACTER*1
*          Specifies the form the system of equations:
*          = 'N':  A * X = B  (No transpose)
*          = 'T':  A**T * X = B  (Transpose)
*          = 'C':  A**H * X = B  (Conjugate transpose = Transpose)
*
*  DIAG    (input) CHARACTER*1
*          = 'N':  A is non-unit triangular;
*          = 'U':  A is unit triangular.
*
*  N       (input) INTEGER
*          The order of the matrix A.  N >= 0.
*
*  KD      (input) INTEGER
*          The number of superdiagonals or subdiagonals of the
*          triangular band matrix A.  KD >= 0.
*
*  NRHS    (input) INTEGER
*          The number of right hand sides, i.e., the number of columns
*          of the matrix B.  NRHS >= 0.
*
*  AB      (input) DOUBLE PRECISION array, dimension (LDAB,N)
*          The upper or lower triangular band matrix A, stored in the
*          first kd+1 rows of AB.  The j-th column of A is stored
*          in the j-th column of the array AB as follows:
*          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
*          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
*          If DIAG = 'U', the diagonal elements of A are not referenced
*          and are assumed to be 1.
*
*  LDAB    (input) INTEGER
*          The leading dimension of the array AB.  LDAB >= KD+1.
*
*  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
*          On entry, the right hand side matrix B.
*          On exit, if INFO = 0, the solution matrix X.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B.  LDB >= max(1,N).
*
*  INFO    (output) INTEGER
*          = 0:  successful exit
*          < 0:  if INFO = -i, the i-th argument had an illegal value
*          > 0:  if INFO = i, the i-th diagonal element of A is zero,
*                indicating that the matrix is singular and the
*                solutions X have not been computed.
*

*  =====================================================================
*

```
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