LAPACK 3.12.0
LAPACK: Linear Algebra PACKage
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◆ dlasy2()

subroutine dlasy2 ( logical ltranl,
logical ltranr,
integer isgn,
integer n1,
integer n2,
double precision, dimension( ldtl, * ) tl,
integer ldtl,
double precision, dimension( ldtr, * ) tr,
integer ldtr,
double precision, dimension( ldb, * ) b,
integer ldb,
double precision scale,
double precision, dimension( ldx, * ) x,
integer ldx,
double precision xnorm,
integer info )

DLASY2 solves the Sylvester matrix equation where the matrices are of order 1 or 2.

Download DLASY2 + dependencies [TGZ] [ZIP] [TXT]

Purpose:
!>
!> DLASY2 solves for the N1 by N2 matrix X, 1 <= N1,N2 <= 2, in
!>
!>        op(TL)*X + ISGN*X*op(TR) = SCALE*B,
!>
!> where TL is N1 by N1, TR is N2 by N2, B is N1 by N2, and ISGN = 1 or
!> -1.  op(T) = T or T**T, where T**T denotes the transpose of T.
!>
Parameters
[in]LTRANL
!>          LTRANL is LOGICAL
!>          On entry, LTRANL specifies the op(TL):
!>             = .FALSE., op(TL) = TL,
!>             = .TRUE., op(TL) = TL**T.
!>
[in]LTRANR
!>          LTRANR is LOGICAL
!>          On entry, LTRANR specifies the op(TR):
!>            = .FALSE., op(TR) = TR,
!>            = .TRUE., op(TR) = TR**T.
!>
[in]ISGN
!>          ISGN is INTEGER
!>          On entry, ISGN specifies the sign of the equation
!>          as described before. ISGN may only be 1 or -1.
!>
[in]N1
!>          N1 is INTEGER
!>          On entry, N1 specifies the order of matrix TL.
!>          N1 may only be 0, 1 or 2.
!>
[in]N2
!>          N2 is INTEGER
!>          On entry, N2 specifies the order of matrix TR.
!>          N2 may only be 0, 1 or 2.
!>
[in]TL
!>          TL is DOUBLE PRECISION array, dimension (LDTL,2)
!>          On entry, TL contains an N1 by N1 matrix.
!>
[in]LDTL
!>          LDTL is INTEGER
!>          The leading dimension of the matrix TL. LDTL >= max(1,N1).
!>
[in]TR
!>          TR is DOUBLE PRECISION array, dimension (LDTR,2)
!>          On entry, TR contains an N2 by N2 matrix.
!>
[in]LDTR
!>          LDTR is INTEGER
!>          The leading dimension of the matrix TR. LDTR >= max(1,N2).
!>
[in]B
!>          B is DOUBLE PRECISION array, dimension (LDB,2)
!>          On entry, the N1 by N2 matrix B contains the right-hand
!>          side of the equation.
!>
[in]LDB
!>          LDB is INTEGER
!>          The leading dimension of the matrix B. LDB >= max(1,N1).
!>
[out]SCALE
!>          SCALE is DOUBLE PRECISION
!>          On exit, SCALE contains the scale factor. SCALE is chosen
!>          less than or equal to 1 to prevent the solution overflowing.
!>
[out]X
!>          X is DOUBLE PRECISION array, dimension (LDX,2)
!>          On exit, X contains the N1 by N2 solution.
!>
[in]LDX
!>          LDX is INTEGER
!>          The leading dimension of the matrix X. LDX >= max(1,N1).
!>
[out]XNORM
!>          XNORM is DOUBLE PRECISION
!>          On exit, XNORM is the infinity-norm of the solution.
!>
[out]INFO
!>          INFO is INTEGER
!>          On exit, INFO is set to
!>             0: successful exit.
!>             1: TL and TR have too close eigenvalues, so TL or
!>                TR is perturbed to get a nonsingular equation.
!>          NOTE: In the interests of speed, this routine does not
!>                check the inputs for errors.
!>
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.