Are there any rules for omitting the * operator in CAS

[webmasterpdx]

???
e.g. int(e^(-ax),x,0,infinity)
vs
int(e^(-a*x),x,0,infinity)

The first version gives a result of infinity, while the second is 1/a for RE(a)>0.

I've noticed that sometimes ax is accepted as a*x and since CAS doesn't report errors when an undefined variable is used, it's often hard to catch these kinds of errors.
Are there any rules for when ax is permissable over a*x ?
Thx
-Donald

[parisse]

ax can never be used for a*x inside the CAS, because otherwise variable names would be restricted to one letter.
You can omit * in situations like number*variable: 2x is ok.
If you don't know, write a * : it won't hurt if you enter 2*x, and this is a good habit if you are going to program.