Question

急求MATLAB解常微分方程组数值解的解决方案

我用MATLAB7.4的ODE45解四元常微分方程组，程序如下：

function xdot = eqx(t,x);

m = 0.5;
alf = 0.15;
K=[0.0265,0.1,0.15];
Q=20;
a=0.8;

xdot = zeros(4,1);
xdot(1) = Q - alf * x(1) * x(2) + a*m*(x(1)+x(2)+x(3)+x(4));
xdot(2) = x(2) * (-m + K(2) * alf * x(1) - alf * x(3));
xdot(3) = x(3) * (-m + K(3) * alf * x(2) - alf * x(4));
xdot(4) = x(4) * (-m + K(4) * alf * x(3));

执行
>> [t,x] = ode45('eqx',[0 0.2] , [0 125] , [0 12.5e+3] , [0 0.2e+3]);
后得到如下出错信息，怎么办啊？

??? Error using ==> funfun\private\odearguments at 44
Correct syntax is ode45(EQX,tspan,y0,options).

Error in ==> ode45 at 173
[neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...

Answer 1

调用方法：[t,x] = ode45(@eqx,[0 0.2] , [0 1 1.1 1.25])x的初始值写在一个矩阵里．
另外你的函数定义时K少一维K(4),我给你加了一个：
function xdot = eqx(t,x);

m = 0.5;
alf = 0.15;
K=[0.0265,0.1,0.15,0.12];
Q=20;
a=0.8;

xdot = zeros(4,1);
xdot(1) = Q - alf * x(1) * x(2) + a*m*(x(1)+x(2)+x(3)+x(4));
xdot(2) = x(2) * (-m + K(2) * alf * x(1) - alf * x(3));
xdot(3) = x(3) * (-m + K(3) * alf * x(2) - alf * x(4));
xdot(4) = x(4) * (-m + K(4) * alf * x(3));

Answer 2

你那个函数里K(4)是多少啊？没有定义