Issue with reality assumption of an implicit function

Why is the assumption on the reality of f(t) lost by assuming real a different function?

Is this a bug?

In fact:

syms f(t) g(t) t real
assume(f(t),'real')
isreal(f(t))
assume(g(t),'real')
isreal(f(t))

returns:

ans =

  logical

   1


ans =

  logical

   0

Any help would be appreciated.

[David Wilson]

My version throws a couple of warnings: I'd look at these first. It seems to me reasonable that a function cannot be restricted easily.

>> syms f(t) g(t) t real
Warning: Can only make assumptions on variable names, not 'f(t)'. 
> In syms (line 248) 
Warning: Can only make assumptions on variable names, not 'g(t)'. 
> In syms (line 248) 

Do you get warnings?

Answer 1

[Marcello Romano]

I solved the problem by using assumeAlso instead of assume, inspired by the answer to https://www.mathworks.com/matlabcentral/answers/78614-how-do-you-declare-a-symbolic-function-of-time-as-a-real-variable

The following code gives no warning and the expected result.

syms f(t) g(t) t 
assumeAlso(f(t),'real')
isreal(f(t))
assumeAlso(g(t),'real')
isreal(f(t))

No idea still why "assume" would not work...

Answer 2

[Marcello Romano]

Yes, I do get the same warning, but no errors, as I would have expected for an erratic result.

For a particular symbolic computation, I am in the need to restrict several functions f(t), g(t), etc. to have real value only.

How to proceed in order to achieve this?