SYNOPSIS

SUBROUTINE DLASR(

SIDE, PIVOT, DIRECT, M, N, C, S, A, LDA )

    

CHARACTER DIRECT, PIVOT, SIDE

    

INTEGER LDA, M, N

    

DOUBLE PRECISION A( LDA, * ), C( * ), S( * )

PURPOSE

DLASR applies a sequence of plane rotations to a real matrix A, from either the left or the right.

When SIDE = 'L', the transformation takes the form


   A := P*A

and when SIDE = 'R', the transformation takes the form


   A := A*P**T

where P is an orthogonal matrix consisting of a sequence of z plane rotations, with z = M when SIDE = 'L' and z = N when SIDE = 'R', and P**T is the transpose of P.

When DIRECT = 'F' (Forward sequence), then


   P = P(z-1) * ... * P(2) * P(1)

and when DIRECT = 'B' (Backward sequence), then


   P = P(1) * P(2) * ... * P(z-1)

where P(k) is a plane rotation matrix defined by the 2-by-2 rotation

   R(k) = (  c(k)  s(k) )

        = ( -s(k)  c(k) ).

When PIVOT = 'V' (Variable pivot), the rotation is performed for the plane (k,k+1), i.e., P(k) has the form


   P(k) = (  1                                            )
          (       ...                                     )
          (              1                                )
          (                   c(k)  s(k)                  )
          (                  -s(k)  c(k)                  )
          (                                1              )
          (                                     ...       )
          (                                            1  )
where R(k) appears as a rank-2 modification to the identity matrix in rows and columns k and k+1.

When PIVOT = 'T' (Top pivot), the rotation is performed for the plane (1,k+1), so P(k) has the form


   P(k) = (  c(k)                    s(k)                 )
          (         1                                     )
          (              ...                              )
          (                     1                         )
          ( -s(k)                    c(k)                 )
          (                                 1             )
          (                                      ...      )
          (                                             1 )
where R(k) appears in rows and columns 1 and k+1.

Similarly, when PIVOT = 'B' (Bottom pivot), the rotation is performed for the plane (k,z), giving P(k) the form


   P(k) = ( 1                                             )
          (      ...                                      )
          (             1                                 )
          (                  c(k)                    s(k) )
          (                         1                     )
          (                              ...              )
          (                                     1         )
          (                 -s(k)                    c(k) )
where R(k) appears in rows and columns k and z. The rotations are performed without ever forming P(k) explicitly.

ARGUMENTS

SIDE (input) CHARACTER*1

Specifies whether the plane rotation matrix P is applied to A on the left or the right. = 'L': Left, compute A := P*A
= 'R': Right, compute A:= A*P**T

PIVOT (input) CHARACTER*1

Specifies the plane for which P(k) is a plane rotation matrix. = 'V': Variable pivot, the plane (k,k+1)
= 'T': Top pivot, the plane (1,k+1)
= 'B': Bottom pivot, the plane (k,z)

DIRECT (input) CHARACTER*1

Specifies whether P is a forward or backward sequence of plane rotations. = 'F': Forward, P = P(z-1)*...*P(2)*P(1)
= 'B': Backward, P = P(1)*P(2)*...*P(z-1)

M (input) INTEGER

The number of rows of the matrix A. If m <= 1, an immediate return is effected.

N (input) INTEGER

The number of columns of the matrix A. If n <= 1, an immediate return is effected.

C (input) DOUBLE PRECISION array, dimension

(M-1) if SIDE = 'L' (N-1) if SIDE = 'R' The cosines c(k) of the plane rotations.

S (input) DOUBLE PRECISION array, dimension

(M-1) if SIDE = 'L' (N-1) if SIDE = 'R' The sines s(k) of the plane rotations. The 2-by-2 plane rotation part of the matrix P(k), R(k), has the form R(k) = ( c(k) s(k) ) ( -s(k) c(k) ).

A (input/output) DOUBLE PRECISION array, dimension (LDA,N)

The M-by-N matrix A. On exit, A is overwritten by P*A if SIDE = 'R' or by A*P**T if SIDE = 'L'.

LDA (input) INTEGER

The leading dimension of the array A. LDA >= max(1,M).