Can someone tell me the meaning of True in output when I use FunctionDomain please? I guess that the meaning of True is: the whole domain (R in the example below). Moreover I'd like to read documentation where I can find an explanation.

FunctionDomain[x, {x}]
(* Out: True *)

Thank you for your willingness.


1 Answer 1


FunctionDomain yields a predicate that's True when the variable is in the domain, so if the domain is unrestricted, the result is simply True.

  • $\begingroup$ Hello @JohnDoty, is there any documentation related to it? $\endgroup$ Commented Feb 20, 2019 at 19:26
  • 1
    $\begingroup$ Just the stuff in the Documentation Center. $\endgroup$
    – John Doty
    Commented Feb 20, 2019 at 20:06
  • 3
    $\begingroup$ The default domain is Reals so True means that Element[x, Reals] evaluates to True $\endgroup$
    – Bob Hanlon
    Commented Feb 21, 2019 at 1:09

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