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Solving a system of ODEs using Runge-Kutta

  1. May 5, 2013 #1
    Using the 4th-order Runge-Kutta method, I have been able to successfully compute the solutions to a coupled pair of two first order differential equations using the following formula:


    When solving systems of ODEs with more than two equations I am unsure if I am properly expanding on the above ideas.

    For example I am trying to find a system of three first order equations:

    da/dx = 2bc + 3a

    db/dx = 4ac +11b

    dc/dx = 5ab + c

    Using and expanding on the equations above would I be correct in the following:

    K0 = 2bc + 3a
    L0 = 4ac +11b
    M0 = 5ab + c

    k1 = 2*(b+(L0)/2)*(c+(M0)/2) + 3*(a+(K0)/2)
    L1 = 4*(a+(K0)/2)*(c+(M0)/2) + 11*(b+(L0)/2)
    M1 = 5*(a+(K0)/2)*(b+(L0)/2) + (c+(M0)/2)

    And so on for K2,L2,M2 etc?

    Hopefully that's fairly readable.

    Is this the correct approach to take or am I making a glaring error somewhere?
  2. jcsd
  3. May 5, 2013 #2


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    Homework Helper

    That looks rignt.

    The basic idea is you first calculate all the "_0" quantities, then use them to calculate all the "_1"'s, etc.

    Trying to do the calculations in a different order won't work, except in special cases and if you get lucky.
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