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c*** 4th order runge-kutta, more or less straight out of |
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c*** numerical recipes |
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subroutine rk4(y,dydx,par,n,x,h,yout,derivs) |
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implicit none |
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|
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integer i, n |
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real y(n),dydx(n),yout(n) |
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real yt(n), par(n), dyt(n) |
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real dym(n) |
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real h, hh, x, xh, h6 |
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external derivs |
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|
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hh=h*0.5 |
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h6=h/6. |
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xh=x+hh |
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|
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do i=1,n |
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yt(i)=y(i)+hh*dydx(i) |
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enddo |
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call derivs(xh,yt,dyt,par,n) |
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do i=1,n |
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yt(i)=y(i)+hh*dyt(i) |
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enddo |
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call derivs(xh,yt,dym,par,n) |
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do i=1,n |
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yt(i)=y(i)+h*dym(i) |
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dym(i)=dyt(i)+dym(i) |
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enddo |
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call derivs(x+h,yt,dyt,par,n) |
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do i=1,n |
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yout(i)=y(i)+h6*(dydx(i)+dyt(i)+2.*dym(i)) |
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enddo |
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|
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return |
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end |
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