sptsv.f.html

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      SUBROUTINE <a name="SPTSV.1"></a><a href="sptsv.f.html#SPTSV.1">SPTSV</a>( N, NRHS, D, E, B, LDB, INFO )
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  -- LAPACK routine (version 3.1) --
</span><span class="comment">*</span><span class="comment">     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
</span><span class="comment">*</span><span class="comment">     November 2006
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     .. Scalar Arguments ..
</span>      INTEGER            INFO, LDB, N, NRHS
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Array Arguments ..
</span>      REAL               B( LDB, * ), D( * ), E( * )
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  Purpose
</span><span class="comment">*</span><span class="comment">  =======
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  <a name="SPTSV.17"></a><a href="sptsv.f.html#SPTSV.1">SPTSV</a> computes the solution to a real system of linear equations
</span><span class="comment">*</span><span class="comment">  A*X = B, where A is an N-by-N symmetric positive definite tridiagonal
</span><span class="comment">*</span><span class="comment">  matrix, and X and B are N-by-NRHS matrices.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  A is factored as A = L*D*L**T, and the factored form of A is then
</span><span class="comment">*</span><span class="comment">  used to solve the system of equations.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  Arguments
</span><span class="comment">*</span><span class="comment">  =========
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  N       (input) INTEGER
</span><span class="comment">*</span><span class="comment">          The order of the matrix A.  N &gt;= 0.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  NRHS    (input) INTEGER
</span><span class="comment">*</span><span class="comment">          The number of right hand sides, i.e., the number of columns
</span><span class="comment">*</span><span class="comment">          of the matrix B.  NRHS &gt;= 0.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  D       (input/output) REAL array, dimension (N)
</span><span class="comment">*</span><span class="comment">          On entry, the n diagonal elements of the tridiagonal matrix
</span><span class="comment">*</span><span class="comment">          A.  On exit, the n diagonal elements of the diagonal matrix
</span><span class="comment">*</span><span class="comment">          D from the factorization A = L*D*L**T.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  E       (input/output) REAL array, dimension (N-1)
</span><span class="comment">*</span><span class="comment">          On entry, the (n-1) subdiagonal elements of the tridiagonal
</span><span class="comment">*</span><span class="comment">          matrix A.  On exit, the (n-1) subdiagonal elements of the
</span><span class="comment">*</span><span class="comment">          unit bidiagonal factor L from the L*D*L**T factorization of
</span><span class="comment">*</span><span class="comment">          A.  (E can also be regarded as the superdiagonal of the unit
</span><span class="comment">*</span><span class="comment">          bidiagonal factor U from the U**T*D*U factorization of A.)
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  B       (input/output) REAL array, dimension (LDB,NRHS)
</span><span class="comment">*</span><span class="comment">          On entry, the N-by-NRHS right hand side matrix B.
</span><span class="comment">*</span><span class="comment">          On exit, if INFO = 0, the N-by-NRHS solution matrix X.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  LDB     (input) INTEGER
</span><span class="comment">*</span><span class="comment">          The leading dimension of the array B.  LDB &gt;= max(1,N).
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  INFO    (output) INTEGER
</span><span class="comment">*</span><span class="comment">          = 0:  successful exit
</span><span class="comment">*</span><span class="comment">          &lt; 0:  if INFO = -i, the i-th argument had an illegal value
</span><span class="comment">*</span><span class="comment">          &gt; 0:  if INFO = i, the leading minor of order i is not
</span><span class="comment">*</span><span class="comment">                positive definite, and the solution has not been
</span><span class="comment">*</span><span class="comment">                computed.  The factorization has not been completed
</span><span class="comment">*</span><span class="comment">                unless i = N.
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">  =====================================================================
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     .. External Subroutines ..
</span>      EXTERNAL           <a name="SPTTRF.64"></a><a href="spttrf.f.html#SPTTRF.1">SPTTRF</a>, <a name="SPTTRS.64"></a><a href="spttrs.f.html#SPTTRS.1">SPTTRS</a>, <a name="XERBLA.64"></a><a href="xerbla.f.html#XERBLA.1">XERBLA</a>
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Intrinsic Functions ..
</span>      INTRINSIC          MAX
<span class="comment">*</span><span class="comment">     ..
</span><span class="comment">*</span><span class="comment">     .. Executable Statements ..
</span><span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     Test the input parameters.
</span><span class="comment">*</span><span class="comment">
</span>      INFO = 0
      IF( N.LT.0 ) THEN
         INFO = -1
      ELSE IF( NRHS.LT.0 ) THEN
         INFO = -2
      ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
         INFO = -6
      END IF
      IF( INFO.NE.0 ) THEN
         CALL <a name="XERBLA.82"></a><a href="xerbla.f.html#XERBLA.1">XERBLA</a>( <span class="string">'<a name="SPTSV.82"></a><a href="sptsv.f.html#SPTSV.1">SPTSV</a> '</span>, -INFO )
         RETURN
      END IF
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     Compute the L*D*L' (or U'*D*U) factorization of A.
</span><span class="comment">*</span><span class="comment">
</span>      CALL <a name="SPTTRF.88"></a><a href="spttrf.f.html#SPTTRF.1">SPTTRF</a>( N, D, E, INFO )
      IF( INFO.EQ.0 ) THEN
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">        Solve the system A*X = B, overwriting B with X.
</span><span class="comment">*</span><span class="comment">
</span>         CALL <a name="SPTTRS.93"></a><a href="spttrs.f.html#SPTTRS.1">SPTTRS</a>( N, NRHS, D, E, B, LDB, INFO )
      END IF
      RETURN
<span class="comment">*</span><span class="comment">
</span><span class="comment">*</span><span class="comment">     End of <a name="SPTSV.97"></a><a href="sptsv.f.html#SPTSV.1">SPTSV</a>
</span><span class="comment">*</span><span class="comment">
</span>      END

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