Clearing variables and functions in Mathematica

[matoma28]

Can anyone help on this?

I am calculating the magnetic susceptibility, essentially a second derivative evaluated at zero magnetic field. The problem is that sometimes Mathematica works perfectly, and sometimes it gives me a "1/0 complexinfinity" error message, with no obvious reason. The problem can arise simply by changing the value of a parameter, but once I return to the parameter values for which it worked fine, Mathematica no longer gives me back the results (it still gives me the same error message). I suspect that it might be an internal problem, i.e. perhaps I need to clear my (user-defined) functions some way.

Chi[Δ_, λ_, θ_, β_] = -D[F, {h, 2}] /. {h -> 0}; (* This works fine *)
Chi[0.4, 0.6, 0.2, β_] (* The problem is here; in particular, when I change the value of θ. Last time the problem occurred because I gave θ a value outside the function (so that I could use in other functions too);

Any help will be greatly appreciated!

 

[Hepth]

Some problems arise in mathematica with evaulation of functions where parts of the function diverge, though overall it may cancel. An example:

Clear[f]
f[x_] = x/(2 - x) + 2/(x - 2)
f[2]
Limit[f[x], x -> 2]

f[2] will give you 1/0 div by zero error, but this function actually goes to -1 at x=2.

Perhaps there is something in your function that diverges at those specific values (at least in part of the equation), but wouldn't overall.

 

[matoma28]

Thanks a lot! It turns out that my function was not analytical at the point of interest, that is why Mathematica did not know which (directional) derivative to give me. Thanks for your response.