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subroutine balanc(nm,n,a,low,igh,scale)
c
integer i,j,k,l,m,n,jj,nm,igh,low,iexc
double precision a(nm,n),scale(n)
double precision c,f,g,r,s,b2,radix
logical noconv
c! purpose
c
c this subroutine balances a real matrix and isolates
c eigenvalues whenever possible.
c! calling sequence
c
c subroutine balanc(nm,n,a,low,igh,scale)
c
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 matrix;
c
c a contains the input matrix to be balanced.
c
c on output:
c
c a contains the balanced matrix;
c
c low and igh are two integers such that a(i,j)
c is equal to zero if
c (1) i is greater than j and
c (2) j=1,...,low-1 or i=igh+1,...,n;
c
c scale contains information determining the
c permutations and scaling factors used.
c
c suppose that the principal submatrix in rows low through igh
c has been balanced, that p(j) denotes the index interchanged
c with j during the permutation step, and that the elements
c of the diagonal matrix used are denoted by d(i,j). then
c scale(j) = p(j), for j = 1,...,low-1
c = d(j,j), j = low,...,igh
c = p(j) j = igh+1,...,n.
c the order in which the interchanges are made is n to igh+1,
c then 1 to low-1.
c
c note that 1 is returned for igh if igh is zero formally.
c
c the algol procedure exc contained in balance appears in
c balanc in line. (note that the algol roles of identifiers
c k,l have been reversed.)
c
c! originator
c
c this subroutine is a translation of the algol procedure balance,
c num. math. 13, 293-304(1969) by parlett and reinsch.
c handbook for auto. comp., vol.ii-linear algebra, 315-326(1971).
c!
c questions and comments should be directed to b. s. garbow,
c applied mathematics division, argonne national laboratory
c
c ------------------------------------------------------------------
c
c :::::::::: radix is a machine dependent parameter specifying
c the base of the machine floating point representation.
data radix/2.0d+0/
c
b2 = radix * radix
k = 1
l = n
go to 100
c :::::::::: in-line procedure for row and
c column exchange ::::::::::
20 scale(m) = j
if (j .eq. m) go to 50
c
do 30 i = 1, l
f = a(i,j)
a(i,j) = a(i,m)
a(i,m) = f
30 continue
c
do 40 i = k, n
f = a(j,i)
a(j,i) = a(m,i)
a(m,i) = f
40 continue
c
50 go to (80,130), iexc
c :::::::::: search for rows isolating an eigenvalue
c and push them down ::::::::::
80 if (l .eq. 1) go to 280
l = l - 1
c :::::::::: for j=l step -1 until 1 do -- ::::::::::
100 do 120 jj = 1, l
j = l + 1 - jj
c
do 110 i = 1, l
if (i .eq. j) go to 110
if (a(j,i) .ne. 0.0d+0) go to 120
110 continue
c
m = l
iexc = 1
go to 20
120 continue
c
go to 140
c :::::::::: search for columns isolating an eigenvalue
c and push them left ::::::::::
130 k = k + 1
c
140 do 170 j = k, l
c
do 150 i = k, l
if (i .eq. j) go to 150
if (a(i,j) .ne. 0.0d+0) go to 170
150 continue
c
m = k
iexc = 2
go to 20
170 continue
c :::::::::: now balance the submatrix in rows k to l ::::::::::
do 180 i = k, l
180 scale(i) = 1.0d+0
c :::::::::: iterative loop for norm reduction ::::::::::
190 noconv = .false.
c
do 270 i = k, l
c = 0.0d+0
r = 0.0d+0
c
do 200 j = k, l
if (j .eq. i) go to 200
c = c + abs(a(j,i))
r = r + abs(a(i,j))
200 continue
c :::::::::: guard against zero c or r due to underflow ::::::::::
if (c .eq. 0.0d+0 .or. r .eq. 0.0d+0) go to 270
g = r / radix
f = 1.0d+0
s = c + r
210 if (c .ge. g) go to 220
f = f * radix
c = c * b2
go to 210
220 g = r * radix
230 if (c .lt. g) go to 240
f = f / radix
c = c / b2
go to 230
c :::::::::: now balance ::::::::::
240 if ((c + r) / f .ge. 0.950d+0 * s) go to 270
g = 1.0d+0 / f
scale(i) = scale(i) * f
noconv = .true.
c
do 250 j = k, n
250 a(i,j) = a(i,j) * g
c
do 260 j = 1, l
260 a(j,i) = a(j,i) * f
c
270 continue
c
if (noconv) go to 190
c
280 low = k
igh = l
return
c :::::::::: last card of balanc ::::::::::
end
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