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I'm new in Mathematica, I want to plot a bifurcation diagram for a complicated Ising system with dynamics that I'm working on, but to get familiar with how to perform it on Mathematica I wish to start with well known mean field approximation

$$m = c\tanh(m)$$

I have no idea how to approach it, what I have so far is:

f1[c_, m_] := c Tanh[m];
f2[m_] := m;
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2 Answers 2

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With your definitions, I assume you want to solve the equation $m = c\tanh(m)$ for $m$ as a function of the parameter $c$. Here is one way of doing this:

mMax = 100;
f1[c_, m_] := c Tanh[m];
f2[m_] := m;    
mIsing[sign_][c_] := 
 Module[{m}, 
  m /. Quiet[NSolve[f1[c, m] == f2[m] && -mMax < m < mMax, m]][[sign]]]

Plot[{mIsing[1][c], mIsing[-1][c]}, {c, 0, 10}]

bifurcation

The function mIsing has c as a variable and also depends on the parameter sign that can be $\pm 1$ for the upper and lower branch (if they exist). The main part id the numerical solution of the equation in NSolve.

I enclose the solver in a Module to make sure the variable m we're solving for is properly localized. The Quiet is in there to suppress a harmless warning message from the solver. Below the bifurcation, the two solutions are just identical to 0.

A faster way to make the plot (if you only want a plot) is this:

ContourPlot[f1[c, m] == f2[m], {c, 0, 10}, {m, -10, 10}]

bifurc2

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  • $\begingroup$ Actually, zero is a solution for all values of c as shown in the ContourPlot; your approach with Plot leaves off the third solution. Recommend generating data and using ListLinePlot to show all three solutions. $\endgroup$
    – Bob Hanlon
    Oct 11, 2015 at 18:04
  • $\begingroup$ @BobHanlon Yes, I assumed we only want the nonzero solutions after the bifurcation - this illustrates how to select particular branches if desired. That depends on the goal of the question, which is left open... $\endgroup$
    – Jens
    Oct 11, 2015 at 18:25
  • $\begingroup$ @Jens ,thanks , this detailed solution was very helpful $\endgroup$
    – jarhead
    Oct 12, 2015 at 5:52
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This really adds nothing to Jens answer (which I have upvoted). I just post it for fun (and to illustrate utility of MeshShading). 

Manipulate[
 Row[{Show[
    Plot[p Tanh[x], {x, -10, 10}, MeshFunctions -> (# - p Tanh[#] &), 
     Mesh -> {{0}}, MeshStyle -> {Red, PointSize[0.02]}, 
     MeshShading -> {Red, Blue}], 
    Plot[x, {x, -10, 10}, PlotStyle -> Dashed], Frame -> True, 
    PlotRange -> {-10, 10}, AspectRatio -> Automatic, 
    ImageSize -> 300],
   ContourPlot[x == r Tanh[x], {r, 0, 10}, {x, -10, 10}, 
    ImageSize -> 300, GridLines -> {{p}, None}, 
    MeshFunctions -> (#1 &), Mesh -> {{p}}, 
    MeshStyle -> {Red, PointSize[0.02]}, PerformanceGoal -> "Quality"]
   }], {p, 0, 10}]

enter image description here

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  • $\begingroup$ Clever. Wonder whether these can be saved as apps for the IPAD....pretty sure these visuals will encourage more students to take maths & physics. $\endgroup$
    – thils
    Oct 12, 2015 at 3:18
  • $\begingroup$ @thils thank you...Mathematica is a wonderful tool to aid understanding...cannot answer iPad question...not enough fun and play in teaching and learning...MSE has taught me a lot so l like to share fun...sometimes I am over the top...so thank you for kind words :-) $\endgroup$
    – ubpdqn
    Oct 12, 2015 at 3:26
  • $\begingroup$ reason I mention IPAD is bcos the schools want students to get one so it is easier to test students for several subjects... $\endgroup$
    – thils
    Oct 12, 2015 at 3:42
  • $\begingroup$ @ubpdqn ,thanks , will work with what you did $\endgroup$
    – jarhead
    Oct 12, 2015 at 5:53

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