LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ slapll()

subroutine slapll ( integer  N,
real, dimension( * )  X,
integer  INCX,
real, dimension( * )  Y,
integer  INCY,
real  SSMIN 
)

SLAPLL measures the linear dependence of two vectors.

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Purpose:
 Given two column vectors X and Y, let

                      A = ( X Y ).

 The subroutine first computes the QR factorization of A = Q*R,
 and then computes the SVD of the 2-by-2 upper triangular matrix R.
 The smaller singular value of R is returned in SSMIN, which is used
 as the measurement of the linear dependency of the vectors X and Y.
Parameters
[in]N
          N is INTEGER
          The length of the vectors X and Y.
[in,out]X
          X is REAL array,
                         dimension (1+(N-1)*INCX)
          On entry, X contains the N-vector X.
          On exit, X is overwritten.
[in]INCX
          INCX is INTEGER
          The increment between successive elements of X. INCX > 0.
[in,out]Y
          Y is REAL array,
                         dimension (1+(N-1)*INCY)
          On entry, Y contains the N-vector Y.
          On exit, Y is overwritten.
[in]INCY
          INCY is INTEGER
          The increment between successive elements of Y. INCY > 0.
[out]SSMIN
          SSMIN is REAL
          The smallest singular value of the N-by-2 matrix A = ( X Y ).
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 101 of file slapll.f.

102 *
103 * -- LAPACK auxiliary routine --
104 * -- LAPACK is a software package provided by Univ. of Tennessee, --
105 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
106 *
107 * .. Scalar Arguments ..
108  INTEGER INCX, INCY, N
109  REAL SSMIN
110 * ..
111 * .. Array Arguments ..
112  REAL X( * ), Y( * )
113 * ..
114 *
115 * =====================================================================
116 *
117 * .. Parameters ..
118  REAL ZERO, ONE
119  parameter( zero = 0.0e+0, one = 1.0e+0 )
120 * ..
121 * .. Local Scalars ..
122  REAL A11, A12, A22, C, SSMAX, TAU
123 * ..
124 * .. External Functions ..
125  REAL SDOT
126  EXTERNAL sdot
127 * ..
128 * .. External Subroutines ..
129  EXTERNAL saxpy, slarfg, slas2
130 * ..
131 * .. Executable Statements ..
132 *
133 * Quick return if possible
134 *
135  IF( n.LE.1 ) THEN
136  ssmin = zero
137  RETURN
138  END IF
139 *
140 * Compute the QR factorization of the N-by-2 matrix ( X Y )
141 *
142  CALL slarfg( n, x( 1 ), x( 1+incx ), incx, tau )
143  a11 = x( 1 )
144  x( 1 ) = one
145 *
146  c = -tau*sdot( n, x, incx, y, incy )
147  CALL saxpy( n, c, x, incx, y, incy )
148 *
149  CALL slarfg( n-1, y( 1+incy ), y( 1+2*incy ), incy, tau )
150 *
151  a12 = y( 1 )
152  a22 = y( 1+incy )
153 *
154 * Compute the SVD of 2-by-2 Upper triangular matrix.
155 *
156  CALL slas2( a11, a12, a22, ssmin, ssmax )
157 *
158  RETURN
159 *
160 * End of SLAPLL
161 *
subroutine slas2(F, G, H, SSMIN, SSMAX)
SLAS2 computes singular values of a 2-by-2 triangular matrix.
Definition: slas2.f:107
subroutine slarfg(N, ALPHA, X, INCX, TAU)
SLARFG generates an elementary reflector (Householder matrix).
Definition: slarfg.f:106
real function sdot(N, SX, INCX, SY, INCY)
SDOT
Definition: sdot.f:82
subroutine saxpy(N, SA, SX, INCX, SY, INCY)
SAXPY
Definition: saxpy.f:89
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