DPFTRI(3)      LAPACK routine of NEC Numeric Library Collection      DPFTRI(3)



NAME
       DPFTRI

SYNOPSIS
       SUBROUTINE DPFTRI (TRANSR, UPLO, N, A, INFO)



PURPOSE
            DPFTRI 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 DPFTRF.




ARGUMENTS
           TRANSR    (input)
                     TRANSR is CHARACTER*1
                     = 'N':  The Normal TRANSR of RFP A is stored;
                     = 'T':  The Transpose TRANSR of RFP A is stored.

           UPLO      (input)
                     UPLO is CHARACTER*1
                     = 'U':  Upper triangle of A is stored;
                     = 'L':  Lower triangle of A is stored.

           N         (input)
                     N is INTEGER
                     The order of the matrix A.  N >= 0.

           A         (input/output)
                     A is DOUBLE PRECISION array, dimension ( N*(N+1)/2 )
                     On entry, the symmetric matrix A in RFP format. RFP format is
                     described by TRANSR, UPLO, and N as follows: If TRANSR = 'N'
                     then RFP A is (0:N,0:k-1) when N is even; k=N/2. RFP A is
                     (0:N-1,0:k) when N is odd; k=N/2. IF TRANSR = 'T' then RFP is
                     the transpose of RFP A as defined when
                     TRANSR = 'N'. The contents of RFP A are defined by UPLO as
                     follows: If UPLO = 'U' the RFP A contains the nt elements of
                     upper packed A. If UPLO = 'L' the RFP A contains the elements
                     of lower packed A. The LDA of RFP A is (N+1)/2 when TRANSR =
                     'T'. When TRANSR is 'N' the LDA is N+1 when N is even and N
                     is odd. See the Note below for more details.

                     On exit, the symmetric inverse of the original matrix, in the
                     same storage format.

           INFO      (output)
                     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.






FURTHER DETAILS
             We first consider Rectangular Full Packed (RFP) Format when N is
             even. We give an example where N = 6.

                 AP is Upper             AP is Lower

              00 01 02 03 04 05       00
                 11 12 13 14 15       10 11
                    22 23 24 25       20 21 22
                       33 34 35       30 31 32 33
                          44 45       40 41 42 43 44
                             55       50 51 52 53 54 55


             Let TRANSR = 'N'. RFP holds AP as follows:
             For UPLO = 'U' the upper trapezoid A(0:5,0:2) consists of the last
             three columns of AP upper. The lower triangle A(4:6,0:2) consists of
             the transpose of the first three columns of AP upper.
             For UPLO = 'L' the lower trapezoid A(1:6,0:2) consists of the first
             three columns of AP lower. The upper triangle A(0:2,0:2) consists of
             the transpose of the last three columns of AP lower.
             This covers the case N even and TRANSR = 'N'.

                    RFP A                   RFP A

                   03 04 05                33 43 53
                   13 14 15                00 44 54
                   23 24 25                10 11 55
                   33 34 35                20 21 22
                   00 44 45                30 31 32
                   01 11 55                40 41 42
                   02 12 22                50 51 52

             Now let TRANSR = 'T'. RFP A in both UPLO cases is just the
             transpose of RFP A above. One therefore gets:


                      RFP A                   RFP A

                03 13 23 33 00 01 02    33 00 10 20 30 40 50
                04 14 24 34 44 11 12    43 44 11 21 31 41 51
                05 15 25 35 45 55 22    53 54 55 22 32 42 52


             We then consider Rectangular Full Packed (RFP) Format when N is
             odd. We give an example where N = 5.

                AP is Upper                 AP is Lower

              00 01 02 03 04              00
                 11 12 13 14              10 11
                    22 23 24              20 21 22
                       33 34              30 31 32 33
                          44              40 41 42 43 44


             Let TRANSR = 'N'. RFP holds AP as follows:
             For UPLO = 'U' the upper trapezoid A(0:4,0:2) consists of the last
             three columns of AP upper. The lower triangle A(3:4,0:1) consists of
             the transpose of the first two columns of AP upper.
             For UPLO = 'L' the lower trapezoid A(0:4,0:2) consists of the first
             three columns of AP lower. The upper triangle A(0:1,1:2) consists of
             the transpose of the last two columns of AP lower.
             This covers the case N odd and TRANSR = 'N'.

                    RFP A                   RFP A

                   02 03 04                00 33 43
                   12 13 14                10 11 44
                   22 23 24                20 21 22
                   00 33 34                30 31 32
                   01 11 44                40 41 42

             Now let TRANSR = 'T'. RFP A in both UPLO cases is just the
             transpose of RFP A above. One therefore gets:

                      RFP A                   RFP A

                02 12 22 00 01             00 10 20 30 40 50
                03 13 23 33 11             33 11 21 31 41 51
                04 14 24 34 44             43 44 22 32 42 52



LAPACK routine                  31 October 2017                      DPFTRI(3)