LAPACK  3.10.0 LAPACK: Linear Algebra PACKage

## ◆ slanv2()

 subroutine slanv2 ( real A, real B, real C, real D, real RT1R, real RT1I, real RT2R, real RT2I, real CS, real SN )

SLANV2 computes the Schur factorization of a real 2-by-2 nonsymmetric matrix in standard form.

Purpose:
``` SLANV2 computes the Schur factorization of a real 2-by-2 nonsymmetric
matrix in standard form:

[ A  B ] = [ CS -SN ] [ AA  BB ] [ CS  SN ]
[ C  D ]   [ SN  CS ] [ CC  DD ] [-SN  CS ]

where either
1) CC = 0 so that AA and DD are real eigenvalues of the matrix, or
2) AA = DD and BB*CC < 0, so that AA + or - sqrt(BB*CC) are complex
conjugate eigenvalues.```
Parameters
 [in,out] A ` A is REAL` [in,out] B ` B is REAL` [in,out] C ` C is REAL` [in,out] D ``` D is REAL On entry, the elements of the input matrix. On exit, they are overwritten by the elements of the standardised Schur form.``` [out] RT1R ` RT1R is REAL` [out] RT1I ` RT1I is REAL` [out] RT2R ` RT2R is REAL` [out] RT2I ``` RT2I is REAL The real and imaginary parts of the eigenvalues. If the eigenvalues are a complex conjugate pair, RT1I > 0.``` [out] CS ` CS is REAL` [out] SN ``` SN is REAL Parameters of the rotation matrix.```
Further Details:
```  Modified by V. Sima, Research Institute for Informatics, Bucharest,
Romania, to reduce the risk of cancellation errors,
when computing real eigenvalues, and to ensure, if possible, that
abs(RT1R) >= abs(RT2R).```

Definition at line 126 of file slanv2.f.

127 *
128 * -- LAPACK auxiliary routine --
129 * -- LAPACK is a software package provided by Univ. of Tennessee, --
130 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
131 *
132 * .. Scalar Arguments ..
133  REAL A, B, C, CS, D, RT1I, RT1R, RT2I, RT2R, SN
134 * ..
135 *
136 * =====================================================================
137 *
138 * .. Parameters ..
139  REAL ZERO, HALF, ONE, TWO
140  parameter( zero = 0.0e+0, half = 0.5e+0, one = 1.0e+0,
141  \$ two = 2.0e+0 )
142  REAL MULTPL
143  parameter( multpl = 4.0e+0 )
144 * ..
145 * .. Local Scalars ..
146  REAL AA, BB, BCMAX, BCMIS, CC, CS1, DD, EPS, P, SAB,
147  \$ SAC, SCALE, SIGMA, SN1, TAU, TEMP, Z, SAFMIN,
148  \$ SAFMN2, SAFMX2
149  INTEGER COUNT
150 * ..
151 * .. External Functions ..
152  REAL SLAMCH, SLAPY2
153  EXTERNAL slamch, slapy2
154 * ..
155 * .. Intrinsic Functions ..
156  INTRINSIC abs, max, min, sign, sqrt
157 * ..
158 * .. Executable Statements ..
159 *
160  safmin = slamch( 'S' )
161  eps = slamch( 'P' )
162  safmn2 = slamch( 'B' )**int( log( safmin / eps ) /
163  \$ log( slamch( 'B' ) ) / two )
164  safmx2 = one / safmn2
165  IF( c.EQ.zero ) THEN
166  cs = one
167  sn = zero
168 *
169  ELSE IF( b.EQ.zero ) THEN
170 *
171 * Swap rows and columns
172 *
173  cs = zero
174  sn = one
175  temp = d
176  d = a
177  a = temp
178  b = -c
179  c = zero
180 *
181  ELSE IF( (a-d).EQ.zero .AND. sign( one, b ).NE.
182  \$ sign( one, c ) ) THEN
183  cs = one
184  sn = zero
185 *
186  ELSE
187 *
188  temp = a - d
189  p = half*temp
190  bcmax = max( abs( b ), abs( c ) )
191  bcmis = min( abs( b ), abs( c ) )*sign( one, b )*sign( one, c )
192  scale = max( abs( p ), bcmax )
193  z = ( p / scale )*p + ( bcmax / scale )*bcmis
194 *
195 * If Z is of the order of the machine accuracy, postpone the
196 * decision on the nature of eigenvalues
197 *
198  IF( z.GE.multpl*eps ) THEN
199 *
200 * Real eigenvalues. Compute A and D.
201 *
202  z = p + sign( sqrt( scale )*sqrt( z ), p )
203  a = d + z
204  d = d - ( bcmax / z )*bcmis
205 *
206 * Compute B and the rotation matrix
207 *
208  tau = slapy2( c, z )
209  cs = z / tau
210  sn = c / tau
211  b = b - c
212  c = zero
213 *
214  ELSE
215 *
216 * Complex eigenvalues, or real (almost) equal eigenvalues.
217 * Make diagonal elements equal.
218 *
219  count = 0
220  sigma = b + c
221  10 CONTINUE
222  count = count + 1
223  scale = max( abs(temp), abs(sigma) )
224  IF( scale.GE.safmx2 ) THEN
225  sigma = sigma * safmn2
226  temp = temp * safmn2
227  IF (count .LE. 20)
228  \$ GOTO 10
229  END IF
230  IF( scale.LE.safmn2 ) THEN
231  sigma = sigma * safmx2
232  temp = temp * safmx2
233  IF (count .LE. 20)
234  \$ GOTO 10
235  END IF
236  p = half*temp
237  tau = slapy2( sigma, temp )
238  cs = sqrt( half*( one+abs( sigma ) / tau ) )
239  sn = -( p / ( tau*cs ) )*sign( one, sigma )
240 *
241 * Compute [ AA BB ] = [ A B ] [ CS -SN ]
242 * [ CC DD ] [ C D ] [ SN CS ]
243 *
244  aa = a*cs + b*sn
245  bb = -a*sn + b*cs
246  cc = c*cs + d*sn
247  dd = -c*sn + d*cs
248 *
249 * Compute [ A B ] = [ CS SN ] [ AA BB ]
250 * [ C D ] [-SN CS ] [ CC DD ]
251 *
252  a = aa*cs + cc*sn
253  b = bb*cs + dd*sn
254  c = -aa*sn + cc*cs
255  d = -bb*sn + dd*cs
256 *
257  temp = half*( a+d )
258  a = temp
259  d = temp
260 *
261  IF( c.NE.zero ) THEN
262  IF( b.NE.zero ) THEN
263  IF( sign( one, b ).EQ.sign( one, c ) ) THEN
264 *
265 * Real eigenvalues: reduce to upper triangular form
266 *
267  sab = sqrt( abs( b ) )
268  sac = sqrt( abs( c ) )
269  p = sign( sab*sac, c )
270  tau = one / sqrt( abs( b+c ) )
271  a = temp + p
272  d = temp - p
273  b = b - c
274  c = zero
275  cs1 = sab*tau
276  sn1 = sac*tau
277  temp = cs*cs1 - sn*sn1
278  sn = cs*sn1 + sn*cs1
279  cs = temp
280  END IF
281  ELSE
282  b = -c
283  c = zero
284  temp = cs
285  cs = -sn
286  sn = temp
287  END IF
288  END IF
289  END IF
290 *
291  END IF
292 *
293 * Store eigenvalues in (RT1R,RT1I) and (RT2R,RT2I).
294 *
295  rt1r = a
296  rt2r = d
297  IF( c.EQ.zero ) THEN
298  rt1i = zero
299  rt2i = zero
300  ELSE
301  rt1i = sqrt( abs( b ) )*sqrt( abs( c ) )
302  rt2i = -rt1i
303  END IF
304  RETURN
305 *
306 * End of SLANV2
307 *
real function slapy2(X, Y)
SLAPY2 returns sqrt(x2+y2).
Definition: slapy2.f:63
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
Here is the caller graph for this function: