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subroutine reduc(nm,n,a,b,dl,ierr)
c
integer i,j,k,n,i1,j1,nm,nn,ierr
double precision a(nm,n),b(nm,n),dl(n)
double precision x,y
c
c this subroutine is a translation of the algol procedure reduc1,
c num. math. 11, 99-110(1968) by martin and wilkinson.
c handbook for auto. comp., vol.ii-linear algebra, 303-314(1971).
c
c this subroutine reduces the generalized symmetric eigenproblem
c ax=(lambda)bx, where b is positive definite, to the standard
c symmetric eigenproblem using the cholesky factorization of b.
c
c on input
c
c nm must be set to the row dimension of two-dimensional
c array parameters as declared in the calling program
c dimension statement.
c
c n is the order of the matrices a and b. if the cholesky
c factor l of b is already available, n should be prefixed
c with a minus sign.
c
c a and b contain the real symmetric input matrices. only the
c full upper triangles of the matrices need be supplied. if
c n is negative, the strict lower triangle of b contains,
c instead, the strict lower triangle of its cholesky factor l.
c
c dl contains, if n is negative, the diagonal elements of l.
c
c on output
c
c a contains in its full lower triangle the full lower triangle
c of the symmetric matrix derived from the reduction to the
c standard form. the strict upper triangle of a is unaltered.
c
c b contains in its strict lower triangle the strict lower
c triangle of its cholesky factor l. the full upper
c triangle of b is unaltered.
c
c dl contains the diagonal elements of l.
c
c ierr is set to
c zero for normal return,
c 7*n+1 if b is not positive definite.
c
c questions and comments should be directed to burton s. garbow,
c mathematics and computer science div, argonne national laboratory
c
c this version dated august 1983.
c
c ------------------------------------------------------------------
c
ierr = 0
nn = iabs(n)
if (n .lt. 0) go to 100
c .......... form l in the arrays b and dl ..........
do 80 i = 1, n
i1 = i - 1
c
do 80 j = i, n
x = b(i,j)
if (i .eq. 1) go to 40
c
do 20 k = 1, i1
20 x = x - b(i,k) * b(j,k)
c
40 if (j .ne. i) go to 60
if (x .le. 0.0d0) go to 1000
y = dsqrt(x)
dl(i) = y
go to 80
60 b(j,i) = x / y
80 continue
c .......... form the transpose of the upper triangle of inv(l)*a
c in the lower triangle of the array a ..........
100 do 200 i = 1, nn
i1 = i - 1
y = dl(i)
c
do 200 j = i, nn
x = a(i,j)
if (i .eq. 1) go to 180
c
do 160 k = 1, i1
160 x = x - b(i,k) * a(j,k)
c
180 a(j,i) = x / y
200 continue
c .......... pre-multiply by inv(l) and overwrite ..........
do 300 j = 1, nn
j1 = j - 1
c
do 300 i = j, nn
x = a(i,j)
if (i .eq. j) go to 240
i1 = i - 1
c
do 220 k = j, i1
220 x = x - a(k,j) * b(i,k)
c
240 if (j .eq. 1) go to 280
c
do 260 k = 1, j1
260 x = x - a(j,k) * b(i,k)
c
280 a(i,j) = x / dl(i)
300 continue
c
go to 1001
c .......... set error -- b is not positive definite ..........
1000 ierr = 7 * n + 1
1001 return
end
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