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Eigenvalues of a tridiagonal matrix

  1. Dec 28, 2007 #1
    Could some one please explain the logic behind the subroutine tqli(d,e,z) found on pg.1228 of numerical recipes in fortran. For this subroutine, d and e are one dimensional arrays and z is an optional multi-dimensional array(only used if also seeking for eigenvectors of matrix). The 3 modules, and the whole jacobi rotation technique has me confused. Thank you very much.
  2. jcsd
  3. Dec 28, 2007 #2


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    It looks like you're looking at the F90 version...
    Have you looked at the online F77 version that provides some explanation?
  4. Dec 28, 2007 #3
    Are you referring to the explanation from indiana university? If it isn't, then could you please send me the link.
  5. Dec 29, 2007 #4
    Can you post the contents of the subroutine?

    I've actually worked on this problem before - in C, however - but maybe I can help provide some input.
  6. Dec 29, 2007 #5
    I'm actually working on a tridiagonal matrix in which the diagonal term is some forumla in terms of integer n, and the super and sub diagonal terms are equivalent to one another(constant, independent of n). I would like to implement this subroutine into my program. I would prefer an explanation in f90, but if you can explain it in f77 then thats fine. Here is the f77 program: http://www.physics.louisville.edu/help/nr/bookfpdf/f11-3.pdf [Broken] (on pg.474)

    I am not too skilled in f77, so could you please be explicit in your explanation. Thank you for your time.
    Here is the subroutine in f90, where the 3 modules are easily accessible on the net:

    SUBROUTINE tqli(d,e,z)

    USE nrtype; USE nrutil, ONLY : assert_eq, nrerror
    USE nr, ONLY : pythag


    REAL(sp), DIMENSION(:), INTENT(inout) :: d, e
    !REAL(sp), DIMENSION(:,:), OPTIONAL, INTENT(inout) :: z

    INTEGER(i4b) :: i, iter, l, m, n, ndum
    REAL(sp) :: b, c, dd, f, g, p, r, s
    REAL(sp), DIMENSION(SIZE(e)) :: ff

    n = assert_eq(SIZE(d), SIZE(e), 'tqli: n')
    !IF(PRESENT(z)) ndum=assert_eq(n, SIZE(z, 1), SIZE(z, 2), 'tqli: ndum')
    DO l=1, n
    iterate: DO
    DO m=l, n-1
    dd=ABS(d(m)) + ABS(d(m+1))
    IF(ABS(e(m))+dd == dd) EXIT
    END DO
    IF (m==l) EXIT iterate
    IF(iter == 30) CALL nrerror('too many iterations in tqli')
    iter = iter+1
    r = pythag(g, 1.0_sp)
    g = d(m) - d(l) + e(l) / (g + SIGN(r, g))
    s = 1.0
    c = 1.0
    p = 0.0
    DO i = m-1, l, -1
    f = s*e(i)
    b = c*e(i)
    r = pythag(f, g)
    e(i+1) = r
    IF(r == 0.0) THEN
    d(i+1) = d(i+1) - p
    e(m) = 0.0
    CYCLE iterate
    END IF
    s = f/r
    c = g/r
    g = d(i + 1) - p
    r = (d(i) - g) * s + 2.0_sp * c * b
    p = s * r
    d(i + 1) = g + p
    g = c * r - b
    !ff(1:n) = z(1:n,i+1)
    !z(1:n, i+1) = s*z(1:n,i) + c*ff(1:n)
    !z(1:n,i) = c*z(1:n, i) - s*ff(1:n)
    !END IF
    END DO
    d(l) = d(l) - p
    e(l) = g
    e(m) = 0.0
    END DO iterate
    END DO

    !I have commented z out, since i'm only interested in the eigenvalues of this matrix.
    Last edited by a moderator: May 3, 2017
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