# real tridiagonal equation solver
subroutine rtris( n, endl, a, b, c, endr, d, q)
integer i,        n
real q(n), d(n), a, b, c, den, endl, endr
temporary real f(n), e(n)
e(1) = -a/endl;   f(1) = d(1)/endl
do i= 2, n-1 {
	den = b+c*e(i-1);   e(i) = -a/den;   f(i) = (d(i)-c*f(i-1))/den
	}
q(n) = (d(n)-c*f(n-1)) / (endr+c*e(n-1))
do i= n-1, 1, -1
	q(i) = e(i) * q(i+1) + f(i)
return;		end