RK4TI:

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   Fourth order Runge-Kutta integration. RHS is time-independent (TI).

   The right-hand-side function "rhs" computes the state derivative. 
   "rhs" is a function handle with 2 inputs, evaluated as:

   rhs(x,a)
 
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   Form:
   x = RK4TI( rhs, x, h, a )
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   Inputs
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   rhs        (:)     Right-hand side function handle
   x         (N,1)    State (column vector)
   h          (1)     Independent variable step
   a          (:)     Input provided to rhs function

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   Outputs
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   x         (N,1)    Updated state

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