LAPACK  3.10.1
LAPACK: Linear Algebra PACKage

◆ spptri()

subroutine spptri ( character  UPLO,
integer  N,
real, dimension( * )  AP,
integer  INFO 
)

SPPTRI

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Purpose:
 SPPTRI computes the inverse of a real symmetric positive definite
 matrix A using the Cholesky factorization A = U**T*U or A = L*L**T
 computed by SPPTRF.
Parameters
[in]UPLO
          UPLO is CHARACTER*1
          = 'U':  Upper triangular factor is stored in AP;
          = 'L':  Lower triangular factor is stored in AP.
[in]N
          N is INTEGER
          The order of the matrix A.  N >= 0.
[in,out]AP
          AP is REAL array, dimension (N*(N+1)/2)
          On entry, the triangular factor U or L from the Cholesky
          factorization A = U**T*U or A = L*L**T, packed columnwise as
          a linear array.  The j-th column of U or L is stored in the
          array AP as follows:
          if UPLO = 'U', AP(i + (j-1)*j/2) = U(i,j) for 1<=i<=j;
          if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = L(i,j) for j<=i<=n.

          On exit, the upper or lower triangle of the (symmetric)
          inverse of A, overwriting the input factor U or L.
[out]INFO
          INFO is INTEGER
          = 0:  successful exit
          < 0:  if INFO = -i, the i-th argument had an illegal value
          > 0:  if INFO = i, the (i,i) element of the factor U or L is
                zero, and the inverse could not be computed.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 92 of file spptri.f.

93 *
94 * -- LAPACK computational routine --
95 * -- LAPACK is a software package provided by Univ. of Tennessee, --
96 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
97 *
98 * .. Scalar Arguments ..
99  CHARACTER UPLO
100  INTEGER INFO, N
101 * ..
102 * .. Array Arguments ..
103  REAL AP( * )
104 * ..
105 *
106 * =====================================================================
107 *
108 * .. Parameters ..
109  REAL ONE
110  parameter( one = 1.0e+0 )
111 * ..
112 * .. Local Scalars ..
113  LOGICAL UPPER
114  INTEGER J, JC, JJ, JJN
115  REAL AJJ
116 * ..
117 * .. External Functions ..
118  LOGICAL LSAME
119  REAL SDOT
120  EXTERNAL lsame, sdot
121 * ..
122 * .. External Subroutines ..
123  EXTERNAL sscal, sspr, stpmv, stptri, xerbla
124 * ..
125 * .. Executable Statements ..
126 *
127 * Test the input parameters.
128 *
129  info = 0
130  upper = lsame( uplo, 'U' )
131  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
132  info = -1
133  ELSE IF( n.LT.0 ) THEN
134  info = -2
135  END IF
136  IF( info.NE.0 ) THEN
137  CALL xerbla( 'SPPTRI', -info )
138  RETURN
139  END IF
140 *
141 * Quick return if possible
142 *
143  IF( n.EQ.0 )
144  $ RETURN
145 *
146 * Invert the triangular Cholesky factor U or L.
147 *
148  CALL stptri( uplo, 'Non-unit', n, ap, info )
149  IF( info.GT.0 )
150  $ RETURN
151 *
152  IF( upper ) THEN
153 *
154 * Compute the product inv(U) * inv(U)**T.
155 *
156  jj = 0
157  DO 10 j = 1, n
158  jc = jj + 1
159  jj = jj + j
160  IF( j.GT.1 )
161  $ CALL sspr( 'Upper', j-1, one, ap( jc ), 1, ap )
162  ajj = ap( jj )
163  CALL sscal( j, ajj, ap( jc ), 1 )
164  10 CONTINUE
165 *
166  ELSE
167 *
168 * Compute the product inv(L)**T * inv(L).
169 *
170  jj = 1
171  DO 20 j = 1, n
172  jjn = jj + n - j + 1
173  ap( jj ) = sdot( n-j+1, ap( jj ), 1, ap( jj ), 1 )
174  IF( j.LT.n )
175  $ CALL stpmv( 'Lower', 'Transpose', 'Non-unit', n-j,
176  $ ap( jjn ), ap( jj+1 ), 1 )
177  jj = jjn
178  20 CONTINUE
179  END IF
180 *
181  RETURN
182 *
183 * End of SPPTRI
184 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine stptri(UPLO, DIAG, N, AP, INFO)
STPTRI
Definition: stptri.f:117
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79
real function sdot(N, SX, INCX, SY, INCY)
SDOT
Definition: sdot.f:82
subroutine sspr(UPLO, N, ALPHA, X, INCX, AP)
SSPR
Definition: sspr.f:127
subroutine stpmv(UPLO, TRANS, DIAG, N, AP, X, INCX)
STPMV
Definition: stpmv.f:142
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