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     \frac {dx}{dt} = k_1x-k_2xy
     \frac {dy}{dt} = k_3xy-k_4x

    k_1 = 0.02
    k_2 = \frac {0.00004}{day \times no. of foxes}
    k_3 = \frac {0.00004}{day \times no. of rabbits}
    k_1 = 0.04
    x_0 = 500 rabbits
    y_0 = 200 foxes

    Plot the concentration of rabbits and foxes as a function of time for a period of 500 days, and 800 days.

    How do I express k2 and k3 though?

    I have polymath and matlab so I don't need to separate them. It will do everything for me. I just don't know what to write for k2 and k3

    LOL was doing similar thing with MATLAB - predator-prey model am I right? Well I used ode45 and defined the diff equations as a separate function. The syntax can be found on the product help bit. Make sure you store the intial conditions as a matrix. PM if you perhaps want a similar .m file (for epidemic modelling though). Also what do you mean by concentration?
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Updated: September 28, 2010

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