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Determine the equilibrium points of a predator-prey model?

Posted 10 years ago

Hi I am trying to determine the equilibrium points in terms of the nondimensional parameters. the following is the non dimensionalised system:

f[u_, v_] := u (1 - u + (-1 + E^(-u \[Alpha])) v)
g[u_, v_] := v (\[Beta] - E^(-u \[Alpha]) \[Beta] - \[Gamma])

I used the following codes to determine the equilibrium points :

equilibrio = Solve[{f[u, v] == 0, g[u, v] == 0}, {u, v}]

And i get the following error:

'' This system cannot be solved with the methods available to Solve. ''

how can i get the equilibrium points? Please leave your thoughts Thanks Jo

6 Replies

Hi,

it appears that NSolve does a good job:

f[u, v] := u (1 - u + (-1 + E^(-u \[Alpha])) v);
g[u, v] := v (\[Beta] - E^(-u \[Alpha]) \[Beta] - \[CapitalGamma]);
NSolve[{f[u, v] == 0, g[u, v] == 0}, {u, v}, Reals]

gives

enter image description here

Cheers,

M.

POSTED BY: Marco Thiel
POSTED BY: Marco Thiel

I'm getting errors when I try to evaluate both your Manipulate expressions. For he first:

Part::take: Cannot take positions 1 through 3 in sols. >>
ReplaceAll::reps: {sols[[1;;3]]} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>

etc.

For the second:

Select::normal: Nonatomic expression expected at position 1 in Select[sols,!MemberQ[#1[[All,2]],Undefined]&]. >>
ReplaceAll::reps: {sols} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>
ReplaceAll::reps: {sols} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>
ReplaceAll::reps: {!MemberQ[#1[[All,2]],Undefined]&} is neither a list of replacement rules nor a valid dispatch table, and so cannot be used for replacing. >>

etc.

POSTED BY: Murray Eisenberg
POSTED BY: Marco Thiel

You do know that the definition of a function uses the form f[u,v]:= . . . Write g[u,v]:= . . . on a separate line. Also parentheses are not matching.

POSTED BY: S M Blinder
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