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So I want to use the NestList[] function, but i'm not sure how to achieve this for a system of equations like I have here $P_{n+1} = (rP_n +sQ_n)(1 - \frac{P_n + Q_n}{100}), \; Q_{n+1} = P_n$. Does anyone know how to do this?

I have (where x is P and y is Q) 

PP[s_, x_, y_] := (rx + s y) (1 - (x + y)/100)
QQ[x_] := x

where r is a constant and s is a variable. This means the equilibrium of this system will depend on s.

but don't know how to set up the NestList[] function for this. If this was one function I would do something like NestList[f,x,5]. So in the end I want to have a list of coordinates.

As another example of what I want, see the first example in this

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  • $\begingroup$ What do you mean by (where x is P and y is Q)? $\endgroup$
    – Dr. belisarius
    Mar 21, 2016 at 15:18
  • $\begingroup$ @Dr.belisarius Sorry, it seems I had neglected to include the original system I was given. I meant that I am using the symbol $x$ instead of $P$ and the symbol $y$ instead of $Q$ for my Mathematica code. It doesn't really matter I suppose. $\endgroup$
    – Ozera
    Mar 21, 2016 at 15:21
  • $\begingroup$ I'm not sure I understand your question @JasonB. The QQ[] function represents the $Q_{n+1} = P_n$ equation. $\endgroup$
    – Ozera
    Mar 21, 2016 at 15:30

3 Answers 3

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You have a recurrence relation, so use RecurrenceTable

RecurrenceTable[{Q[n + 1] == P[n],
  P[n + 1] == (r P[n] + s Q[n]) (1 - (P[n] + Q[n])/100),
  P[0] == 1,
  Q[0] == 1}, {P, Q}, {n, 2}]

(* {{1, 1}, {(49 (r + s))/50, 1}, 
    {(s + 49/50 r (r + s)) (1 + 1/100 (-1 - (49 (r + s))/50)), (49 (r + s))/50}} *)
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  • $\begingroup$ Very neat! I didn't know about the RecurrenceTable command. It seems like what you wrote gives the exact same output as what @Dr.Belisarius wrote. $\endgroup$
    – Ozera
    Mar 21, 2016 at 15:39
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    $\begingroup$ @Ozera Next time ask what you want. And if you don't know what you want, say it. $\endgroup$
    – Dr. belisarius
    Mar 21, 2016 at 15:50
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NestList[Function[{p, q}, {(r p + s q) (1 - (p + q)/100), p}][Sequence @@ #] &, 
        {1, 1}, 2]

(* {{1, 1}, {(49 (r + s))/50, 1}, 
    {(s + 49/50 r (r + s)) (1 + 1/100 (-1 - (49 (r + s))/50)), (49 (r + s))/50}}*)
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    $\begingroup$ Thank you for answering! Do you mind explaining what [Sequence @@ #] & does? I am not familiar with it. $\endgroup$
    – Ozera
    Mar 21, 2016 at 15:31
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mat[r_, s_, n_, i_] :=
 With[{mx = {{r, s}, {1, 0}} - {{1/100, 1/100}, {0, 0}}},
  MatrixPower[mx, n].i]

e.g.

With[{n = Range[0, 4]}, 
 TableForm[Expand@mat[r, s, #, {1, 1}] & /@ n, 
  TableHeadings -> {n, {"P", "Q"}}, TableAlignments -> Center]]

enter image description here

or

DiscretePlot[Evaluate@mat[1, 1, x, {1, 1}], {x, 0, 5}, 
 PlotLegends -> {"P", "Q"}]

enter image description here

Apologies for any errors or misunderstanding.

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