30 Fortran90µÄRunge Kutta×Ó³ÌÐò-II
h2=.5*h ! h/2, for convenience
do k = 1, ns
Xdelta1=V
Vdelta1=Grav(X,M)
Xdelta2=V+h2*Vdelta1
Vdelta2=Grav (X+h2*Xdelta1, M)
Xdelta3=V+h2*Vdelta2
Vdelta3=Grav(X+h2*Xdelta2, M)
Xdelta4=V+h*Vdelta3
Vdelta4=Grav(X+h*Xdelta3,m)
X=X+h/6. (Xdelta1+2*(Xdelta2+Xdelta3)+Xdelta4)
V=V+h/6. (Vdelta1+2*(Vdelta2+Vdelta3)+Vdelta4)
end do
end subroutine runge-kutta
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