c***Euler program from Koonin, Chapter 2
c***modified to take Runge-Kutta step using rk4.f from Numerical Recipes
      dimension y(1),dydx(1)
      external derivs
      n=1
20    print *, 'Enter step size ( .le. 0 to stop)'
      read *,h
      if(h. le. 0.) stop
      nstep=3./h                   !number of steps to reach x=3
      y(1)=1.                      !y(0)=1
      do ix=0,nstep-1              !loop over steps
         x=ix*h                    !last x value
c        y=y+h*func(x,y)
         call derivs(x,y,dydx)
         call rk4(y,dydx,n,x,h,y,derivs)
         yexact = exp(-0.5*(x+h)**2)
         diff=yexact-y(1)   !compare with exact value
         print *, ix,x+h,y,yexact,diff
      enddo
      goto 20                      !start again with new value of h
      end
c******************************************************************
      subroutine derivs(x,y,dydx)
      dimension y(1),dydx(1)
      dydx(1) = -x*y(1)
      return
      end
c******************************************************************