51 lines
1.6 KiB
FortranFixed
51 lines
1.6 KiB
FortranFixed
subroutine eigen_sym_up(N,A,W)
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implicit none
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!
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!compute the eigenvalues and eigenvectors of a symmetrical matrix.
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!A: On entry, A is a symmetrical matrix with its upper triangle filled.
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! on exit, A contains the normalized eigenvectors in its columns.
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!W: contains the eigenvalues in descending order.
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CHARACTER JOBZ, UPLO
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INTEGER INFO, LDA, LWORK, N
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DOUBLE PRECISION A(N, N), W( N ), WORK(3*N-1)
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double precision p
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integer i,j
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JOBZ='V'
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UPLO='U'
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LWORK=3*N-1
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LDA=N
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call DSYEV(JOBZ,UPLO,N,A(1:N,1:N),LDA,W,WORK,LWORK,INFO)
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* INFO (output) INTEGER
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* = 0: successful exit
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* < 0: if INFO = -i, the i-th argument had an illegal value
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* > 0: if INFO = i, the algorithm failed to converge; i
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* off-diagonal elements of an intermediate tridiagonal
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* form did not converge to zero.
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if(INFO.lt.0)then
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write(*,*)'The ',-INFO,
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& 'th argument in DSYEV has an illegal value'
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stop
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endif
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if(INFO.gt.0)then
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write(*,*)'The algorithm failed to converge'
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stop
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endif
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! Change the eigenvalue array from ascending to descending order and rearrange
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! the eigen vectors accordingly.
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!---------------------------------------------
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do i=1,N/2
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p=W(i)
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W(i)=W(N-i+1)
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W(N-i+1)=p
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do j=1,N
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p=A(j,i)
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A(j,i)=A(j,N-i+1)
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A(j,N-i+1)=p
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enddo
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enddo
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!---------------------------------------------
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return
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end |