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I need to solve a system of two ODE's using improved Euler's (Heun) method. Can any one help as I am pretty bad at Mathematica. This is what I've come up with so far 

f[t_, x_, y_] = -4 x - 2 y + cos (t) + 4 sin (t);
g[t_, x_, y_] = 3 x + y - 3 sin (t);
xlist = {0};
ylist = {-1};
h = .1;
n = 10;
For[i = 1, i <= n, i++,
 {
  k1 = h*f[ h*(i - 1), xlist[[i]], ylist[[i]] ],
  m1 = h*g[ h*(i - 1), xlist[[i]], ylist[[i]] ],
  k2 = h*f[h*(i - 0.5), xlist[[i], ylist[[i]] + 0.5 k1],
     m2 = h*g[h*(i - 0.5), xlist[[i], ylist[[i]] + 0.5 m1],
     k3 = h*f[h*(i - 0.5), xlist[[i]], ylist[[i]] + 0.5 k2]],
     m3 = h*g[h*(i - 0.5), xlist[[i], ylist[[i]] + 0.5 m2],
        k4 = h*f[ h*i, xlist[[i]], ylist[[i]] + k3]
           m4 = h*g[ h*i, xlist[[i]], ylist[[i]] + m3]
    AppendTo[xlist, xlist[[i]] + 1/6*(k1 + k2 + k3 + k4)],
    AppendTo[ylist, ylist[[i]] + 1/6*(m1 + m2 + m3_m4)]
    ]]}]
xlist
ylist
J. M.'s missing motivation
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Matt Rueben
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  • One implementation of Heun's method can be found here: https://mathematica.stackexchange.com/questions/45436/runge-kutta-2-on-system/89168#89168 – Michael E2 May 06 '17 at 21:30
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    You should take some time to learn the syntax, how function names are capitalized, when to use brackets, braces, and parentheses. Perhaps start by searching the documents for sin. – Michael E2 May 06 '17 at 21:41
  • Duplicate: https://mathematica.stackexchange.com/questions/48978/runge-kutta-2nd-order-ode-solver – Michael E2 May 07 '17 at 02:38

1 Answers1

1

Toooooo many mistakes and syntax errors

f[t_, x_, y_] := -4 x - 2 y + Cos [t] + 4 Sin [t];
g[t_, x_, y_] := 3 x + y - 3 Sin [t];
xlist = {0};
ylist = {-1};
h = .1;
n = 10;
For[i = 1, i <= n, 
i++, {k1 = h*f[h*(i - 1), xlist[[i]], ylist[[i]]]; 
m1 = h*g[h*(i - 1), xlist[[i]], ylist[[i]]]; 
k2 = h*f[h*(i - 0.5), xlist[[i]], ylist[[i]] + 0.5 k1]; 
m2 = h*g[h*(i - 0.5), xlist[[i]], ylist[[i]] + 0.5 m1]; 
k3 = h*f[h*(i - 0.5), xlist[[i]], ylist[[i]] + 0.5 k2]; 
m3 = h*g[h*(i - 0.5), xlist[[i]], ylist[[i]] + 0.5 m2]; 
k4 = h*f[h*i, xlist[[i]], ylist[[i]] + k3];
m4 = h*g[h*i, xlist[[i]], ylist[[i]] + m3] ; 
AppendTo[xlist, xlist[[i]] + 1/6*(k1 + k2 + k3 + k4)], 
AppendTo[ylist, ylist[[i]] + 1/6*(m1 + m2 + m3 + m4)]}]
xlist
ylist

{0, 0.193667, 0.373655, 0.540596, 0.694934, 0.836957, 0.966821, \ 1.08458, 1.19021, 1.28362, 1.36471}

{-1, -1.08051, -1.14683, -1.20053, -1.24296, -1.27526, -1.2984, \ -1.31319, -1.32032, -1.32039, -1.31388}

done

EDIT
plus a bonus in order to "see" the result

ListPlot[Table[{xlist[[t]], ylist[[t]]}, {t, 1, Length@xlist}]]

enter image description here

and for n=1000 you get a nice attractor

enter image description here

ZaMoC
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    I hope you get an A on this assignment, but I'm afraid that this use of For and AppendTo is bad practice. – Michael E2 May 07 '17 at 02:36