 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ zpttrf()

 subroutine zpttrf ( integer N, double precision, dimension( * ) D, complex*16, dimension( * ) E, integer INFO )

ZPTTRF

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Purpose:
``` ZPTTRF computes the L*D*L**H factorization of a complex Hermitian
positive definite tridiagonal matrix A.  The factorization may also
be regarded as having the form A = U**H *D*U.```
Parameters
 [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in,out] D ``` D is DOUBLE PRECISION array, dimension (N) On entry, the n diagonal elements of the tridiagonal matrix A. On exit, the n diagonal elements of the diagonal matrix D from the L*D*L**H factorization of A.``` [in,out] E ``` E is COMPLEX*16 array, dimension (N-1) On entry, the (n-1) subdiagonal elements of the tridiagonal matrix A. On exit, the (n-1) subdiagonal elements of the unit bidiagonal factor L from the L*D*L**H factorization of A. E can also be regarded as the superdiagonal of the unit bidiagonal factor U from the U**H *D*U factorization of A.``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -k, the k-th argument had an illegal value > 0: if INFO = k, the leading minor of order k is not positive definite; if k < N, the factorization could not be completed, while if k = N, the factorization was completed, but D(N) <= 0.```

Definition at line 91 of file zpttrf.f.

92 *
93 * -- LAPACK computational routine --
94 * -- LAPACK is a software package provided by Univ. of Tennessee, --
95 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
96 *
97 * .. Scalar Arguments ..
98  INTEGER INFO, N
99 * ..
100 * .. Array Arguments ..
101  DOUBLE PRECISION D( * )
102  COMPLEX*16 E( * )
103 * ..
104 *
105 * =====================================================================
106 *
107 * .. Parameters ..
108  DOUBLE PRECISION ZERO
109  parameter( zero = 0.0d+0 )
110 * ..
111 * .. Local Scalars ..
112  INTEGER I, I4
113  DOUBLE PRECISION EII, EIR, F, G
114 * ..
115 * .. External Subroutines ..
116  EXTERNAL xerbla
117 * ..
118 * .. Intrinsic Functions ..
119  INTRINSIC dble, dcmplx, dimag, mod
120 * ..
121 * .. Executable Statements ..
122 *
123 * Test the input parameters.
124 *
125  info = 0
126  IF( n.LT.0 ) THEN
127  info = -1
128  CALL xerbla( 'ZPTTRF', -info )
129  RETURN
130  END IF
131 *
132 * Quick return if possible
133 *
134  IF( n.EQ.0 )
135  \$ RETURN
136 *
137 * Compute the L*D*L**H (or U**H *D*U) factorization of A.
138 *
139  i4 = mod( n-1, 4 )
140  DO 10 i = 1, i4
141  IF( d( i ).LE.zero ) THEN
142  info = i
143  GO TO 30
144  END IF
145  eir = dble( e( i ) )
146  eii = dimag( e( i ) )
147  f = eir / d( i )
148  g = eii / d( i )
149  e( i ) = dcmplx( f, g )
150  d( i+1 ) = d( i+1 ) - f*eir - g*eii
151  10 CONTINUE
152 *
153  DO 20 i = i4 + 1, n - 4, 4
154 *
155 * Drop out of the loop if d(i) <= 0: the matrix is not positive
156 * definite.
157 *
158  IF( d( i ).LE.zero ) THEN
159  info = i
160  GO TO 30
161  END IF
162 *
163 * Solve for e(i) and d(i+1).
164 *
165  eir = dble( e( i ) )
166  eii = dimag( e( i ) )
167  f = eir / d( i )
168  g = eii / d( i )
169  e( i ) = dcmplx( f, g )
170  d( i+1 ) = d( i+1 ) - f*eir - g*eii
171 *
172  IF( d( i+1 ).LE.zero ) THEN
173  info = i + 1
174  GO TO 30
175  END IF
176 *
177 * Solve for e(i+1) and d(i+2).
178 *
179  eir = dble( e( i+1 ) )
180  eii = dimag( e( i+1 ) )
181  f = eir / d( i+1 )
182  g = eii / d( i+1 )
183  e( i+1 ) = dcmplx( f, g )
184  d( i+2 ) = d( i+2 ) - f*eir - g*eii
185 *
186  IF( d( i+2 ).LE.zero ) THEN
187  info = i + 2
188  GO TO 30
189  END IF
190 *
191 * Solve for e(i+2) and d(i+3).
192 *
193  eir = dble( e( i+2 ) )
194  eii = dimag( e( i+2 ) )
195  f = eir / d( i+2 )
196  g = eii / d( i+2 )
197  e( i+2 ) = dcmplx( f, g )
198  d( i+3 ) = d( i+3 ) - f*eir - g*eii
199 *
200  IF( d( i+3 ).LE.zero ) THEN
201  info = i + 3
202  GO TO 30
203  END IF
204 *
205 * Solve for e(i+3) and d(i+4).
206 *
207  eir = dble( e( i+3 ) )
208  eii = dimag( e( i+3 ) )
209  f = eir / d( i+3 )
210  g = eii / d( i+3 )
211  e( i+3 ) = dcmplx( f, g )
212  d( i+4 ) = d( i+4 ) - f*eir - g*eii
213  20 CONTINUE
214 *
215 * Check d(n) for positive definiteness.
216 *
217  IF( d( n ).LE.zero )
218  \$ info = n
219 *
220  30 CONTINUE
221  RETURN
222 *
223 * End of ZPTTRF
224 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
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