I think that the app is behaving correctly. (I don't think the HP48-HP49-HP50G is able to do symbolic manipulations with "program" functions either, for that matter.) Use an explicit function in F2 instead.

Depending on where a "function" is created, it seems to be interpreted differently.

Using DEFINE to create MYFUNC(X), MYFUNC is treated similar to regular non-CAS command, which is quite different from a function created using the CAS screen via:

FUNC(X):=X^2-5;

In this case, FUNC is considered as a function (DOM_FUNC) as well as a CAS program. One cannot use MYFUNC from the CAS to do symbolic calculations such as MYFUNC(a). (This bit is not really an issue -- for me anyway -- as I am sure that it is part of the "CAS vs non-CAS" design.) And all calculations with MYFUNC return non-exact values, again as if MYFUNC were a non-CAS function, as opposed to FUNC which is a CAS function.

**Strangely, if we define (in CAS mode):**

**
f(X):=X^2-5;**

**
then f is a symbolic (DOM_SYMBOLIC) and not a function. It is considered a "program" in that it returns program(X,0,X^2-5). However, it does not show up in the program list, and does not evaluate properly. In the CAS screen, typing f(1) produces an "Error:Invalid Input"**

On the other hand, symbolic functions such as FUNC (explained above) theoretically _should_ work since the calc is able to properly handle CAS-created functions. However, I am unable to get the tangent line feature to work even with CAS-created functions (perhaps again because CAS-created functions are actually still programs).

Now, since f(X) (as defined above) shows up as a symbolic type, I was thinking perhaps this case should work -- except it doesn't either. In fact, the plot shows up as undefined (even using CAS.f(X) in Symbolic View). One-letter functions seem to be treated differently.

*Edited: 4 Oct 2013, 11:48 a.m. *