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NAME
     cgbsv - compute the solution to a complex system  of  linear
     equations  A  *  X  = B, where A is a band matrix of order N
     with KL subdiagonals and KU superdiagonals, and X and B  are
     N-by-NRHS matrices

SYNOPSIS
     SUBROUTINE CGBSV( N, KL, KU, NRHS, AB, LDAB, IPIV,  B,  LDB,
               INFO )

     INTEGER INFO, KL, KU, LDAB, LDB, N, NRHS

     INTEGER IPIV( * )

     COMPLEX AB( LDAB, * ), B( LDB, * )



     #include <sunperf.h>

     void cgbsv(int n, int kl, int ku, int  nrhs,  complex  *cab,
               int  ldab,  int *ipivot, complex *cb, int ldb, int
               *info);

PURPOSE
     CGBSV computes the solution to a complex  system  of  linear
     equations  A  *  X  = B, where A is a band matrix of order N
     with KL subdiagonals and KU superdiagonals, and X and B  are
     N-by-NRHS matrices.

     The LU decomposition with partial pivoting  and  row  inter-
     changes  is used to factor A as A = L * U, where L is a pro-
     duct of permutation and unit lower triangular matrices  with
     KL subdiagonals, and U is upper triangular with KL+KU super-
     diagonals.  The factored form of A is then used to solve the
     system of equations A * X = B.


ARGUMENTS
     N         (input) INTEGER
               The number of linear equations, i.e., the order of
               the matrix A.  N >= 0.

     KL        (input) INTEGER
               The number of subdiagonals within the band  of  A.
               KL >= 0.

     KU        (input) INTEGER
               The number of superdiagonals within the band of A.
               KU >= 0.

     NRHS      (input) INTEGER
               The number of right hand sides, i.e.,  the  number
               of columns of the matrix B.  NRHS >= 0.

     AB        (input/output) COMPLEX array, dimension (LDAB,N)
               On entry, the matrix A in band  storage,  in  rows
               KL+1  to 2*KL+KU+1; rows 1 to KL of the array need
               not be set.  The j-th column of A is stored in the
               j-th   column   of   the   array  AB  as  follows:
               AB(KL+KU+1+i-j,j)   =    A(i,j)    for    max(1,j-
               KU)<=i<=min(N,j+KL)  On  exit, details of the fac-
               torization: U is stored  as  an  upper  triangular
               band matrix with KL+KU superdiagonals in rows 1 to
               KL+KU+1, and the multipliers used during the  fac-
               torization   are   stored   in   rows  KL+KU+2  to
               2*KL+KU+1.  See below for further details.

     LDAB      (input) INTEGER
               The leading dimension of the array  AB.   LDAB  >=
               2*KL+KU+1.

     IPIV      (output) INTEGER array, dimension (N)
               The pivot  indices  that  define  the  permutation
               matrix  P;  row  i  of the matrix was interchanged
               with row IPIV(i).

     B         (input/output) COMPLEX array, dimension (LDB,NRHS)
               On entry, the N-by-NRHS right hand side matrix  B.
               On  exit,  if  INFO  =  0,  the N-by-NRHS 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 ille-
               gal value
               > 0:  if INFO = i, U(i,i) is  exactly  zero.   The
               factorization has been completed, but the factor U
               is exactly singular, and the solution has not been
               computed.

FURTHER DETAILS
     The band storage scheme  is  illustrated  by  the  following
     example, when M = N = 6, KL = 2, KU = 1:
     On entry:                       On exit:

         *    *    *    +    +    +       *    *    *   u14  u25  u36
         *    *    +    +    +    +       *    *   u13  u24  u35  u46
         *   a12  a23  a34  a45  a56      *   u12  u23  u34  u45  u56
        a11  a22  a33  a44  a55  a66     u11  u22  u33  u44  u55  u66
        a21  a32  a43  a54  a65   *      m21  m32  m43  m54  m65   *
        a31  a42  a53  a64   *    *      m31  m42  m53  m64   *    *

     Array elements marked * are not used  by  the  routine;
     elements  marked  +  need  not be set on entry, but are
     required by the routine to store elements of U  because
     of fill-in resulting from the row interchanges.

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