subroutine rk4(y, dydx, x, h, yout, derive)
implicit none
integer i
double precision h, x, dydx(2), y(2), yout(2)
external derive
double precision h6, hh, xh, dym(2), dyt(2), yt(2)
hh = h*0.5d0
h6 = h/6d0
xh = x + hh
do i = 1, 2
yt(i) = y(i) + hh*dydx(i)
call derive(xh, yt, dyt)
yt(i) = y(i) + hh*dyt(i)
call derive(xh, yt, dym)
yt(i) = y(i) + h*dym(i)
dym(i) = dyt(i) + dym(i)
call derive(x+h, yt, dyt)
yout(i) = y(i) + h6*(dydx(i) + dyt(i) + 2.0d0*dym(i))
end do
return
end