subroutine trbak1(nm,n,a,e,m,z)
c
integer i,j,k,l,m,n,nm
double precision a(nm,n),e(n),z(nm,m)
double precision s
c
c this subroutine is a translation of the algol procedure trbak1,
c num. math. 11, 181-195(1968) by martin, reinsch, and wilkinson.
c handbook for auto. comp., vol.ii-linear algebra, 212-226(1971).
c
c this subroutine forms the eigenvectors of a real symmetric
c matrix by back transforming those of the corresponding
c symmetric tridiagonal matrix determined by tred1.
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 information about the orthogonal trans-
c formations used in the reduction by tred1
c in its strict lower triangle.
c
c e contains the subdiagonal elements of the tridiagonal
c matrix in its last n-1 positions. e(1) is arbitrary.
c
c m is the number of eigenvectors to be back transformed.
c
c z contains the eigenvectors to be back transformed
c in its first m columns.
c
c on output
c
c z contains the transformed eigenvectors
c in its first m columns.
c
c note that trbak1 preserves vector euclidean norms.
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
if (m .eq. 0) go to 200
if (n .eq. 1) go to 200
c
do 140 i = 2, n
l = i - 1
if (e(i) .eq. 0.0d0) go to 140
c
do 130 j = 1, m
s = 0.0d0
c
do 110 k = 1, l
110 s = s + a(i,k) * z(k,j)
c .......... divisor below is negative of h formed in tred1.
c double division avoids possible underflow ..........
s = (s / a(i,l)) / e(i)
c
do 120 k = 1, l
120 z(k,j) = z(k,j) + s * a(i,k)
c
130 continue
c
140 continue
c
200 return
end