subroutine spline(x,y,n,yp1,ypn,y2)

! + + + argument definitions + + +
      real x, y, yp1, ypn, y2
      integer n

! + + + local variables
      integer i, nmax, k
      real u, sig, p, qn, un

      parameter (nmax=100)
      dimension x(n),y(n),y2(n),u(nmax)
      if (yp1.GT..99E30) then
        y2(1)=0.
        u(1)=0.
      else
        y2(1)=-0.5
        u(1)=(3./(x(2)-x(1)))*((y(2)-y(1))/(x(2)-x(1))-yp1)
      endif
      do 11 i=2,n-1
        sig=(x(i)-x(i-1))/(x(i+1)-x(i-1))
        p=sig*y2(i-1)+2.
        y2(i)=(sig-1.)/p
        u(i)=(6.*((y(i+1)-y(i))/(x(i+1)-x(i))-(y(i)-y(i-1))             &
     &      /(x(i)-x(i-1)))/(x(i+1)-x(i-1))-sig*u(i-1))/p
11    continue
      if (ypn.GT..99E30) then
        qn=0.
        un=0.
      else
        qn=0.5
        un=(3./(x(n)-x(n-1)))*(ypn-(y(n)-y(n-1))/(x(n)-x(n-1)))
      endif
      y2(n)=(un-qn*u(n-1))/(qn*y2(n-1)+1.)
      do 12 k=n-1,1,-1
        y2(k)=y2(k)*y2(k+1)+u(k)
12    continue
!       Debug lines. Remove after debugging.
!        do i=1,n
!                write(56,*)x(i),y(i),y2(i)
!        end do        
      return
      end