 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ dlapll()

 subroutine dlapll ( integer N, double precision, dimension( * ) X, integer INCX, double precision, dimension( * ) Y, integer INCY, double precision SSMIN )

DLAPLL 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 DOUBLE PRECISION 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 DOUBLE PRECISION 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 DOUBLE PRECISION The smallest singular value of the N-by-2 matrix A = ( X Y ).```

Definition at line 101 of file dlapll.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  DOUBLE PRECISION SSMIN
110 * ..
111 * .. Array Arguments ..
112  DOUBLE PRECISION X( * ), Y( * )
113 * ..
114 *
115 * =====================================================================
116 *
117 * .. Parameters ..
118  DOUBLE PRECISION ZERO, ONE
119  parameter( zero = 0.0d+0, one = 1.0d+0 )
120 * ..
121 * .. Local Scalars ..
122  DOUBLE PRECISION A11, A12, A22, C, SSMAX, TAU
123 * ..
124 * .. External Functions ..
125  DOUBLE PRECISION DDOT
126  EXTERNAL ddot
127 * ..
128 * .. External Subroutines ..
129  EXTERNAL daxpy, dlarfg, dlas2
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 dlarfg( n, x( 1 ), x( 1+incx ), incx, tau )
143  a11 = x( 1 )
144  x( 1 ) = one
145 *
146  c = -tau*ddot( n, x, incx, y, incy )
147  CALL daxpy( n, c, x, incx, y, incy )
148 *
149  CALL dlarfg( 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 dlas2( a11, a12, a22, ssmin, ssmax )
157 *
158  RETURN
159 *
160 * End of DLAPLL
161 *
subroutine dlas2(F, G, H, SSMIN, SSMAX)
DLAS2 computes singular values of a 2-by-2 triangular matrix.
Definition: dlas2.f:107
double precision function ddot(N, DX, INCX, DY, INCY)
DDOT
Definition: ddot.f:82
subroutine daxpy(N, DA, DX, INCX, DY, INCY)
DAXPY
Definition: daxpy.f:89
subroutine dlarfg(N, ALPHA, X, INCX, TAU)
DLARFG generates an elementary reflector (Householder matrix).
Definition: dlarfg.f:106
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