Why is '==' returning true, when used to define equations in NDSolve?

[gianlucaarianna (Archos Argentum)]

Thus far I have my function set up essentially like this:

s = NDSolve[{ y'[t] == k*y[ x ] + m*z[ x ]*a[ x ], a'[ x ] == -c*a[ x ] + b*z[ x ]*y[ x ], y[0] = .3, a[0] =.01}, 
{y,z,a}, {x,0,10}]

(also my variable names have subscripts)

When I hit shift+enter, I get an error message telling me that instead of equations, the program is just returning “True”.
What could I be doing wrong?

Thank you,
Archos

[shenghuiy (Shenghui Yang)]

There are several issues in your code:

1. In NDSolve, all parameters must be specified. Say, k must be a known number here, like - 0.1.
2. I do not see the definition of z or you need one more equation and initial condition for it.

Here is an example modified:

[iliang (Ilian Gachevski)]

Set (=) and Equal (==) are not interchangeable. After an assignment like y[0] = 1, the expression y[0] == 1 evaluates to True and that is no longer an equation, so NDSolve issues an appropriate message.