I'm having a problem:

FreeQ[a, _?(# < 0 &)]

returns True when a is kept symbolic.

Shouldn't it hold somehow, returning a conditional expression. How can I make this happen with FreeQ?

P.S.: I am not interested in alternative functions, I just want to understand this behavior.


closed as off-topic by m_goldberg, corey979, MarcoB, Feyre, C. E. Jan 28 '17 at 19:45

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  • 1
    $\begingroup$ You really ought to be doing FreeQ[a, _?(# < 0 &)]. $\endgroup$ – J. M. is away Jan 27 '17 at 15:09
  • $\begingroup$ You are right (edited), but this doesn't solve the problem. $\endgroup$ – MaPo Jan 27 '17 at 15:13
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    $\begingroup$ I'm voting to close this question as off-topic because the OP is asking for functionality that is not supported given the constraints the OP is putting on the solution. $\endgroup$ – m_goldberg Jan 27 '17 at 18:41

FreeQ[a, _?(a < 0 &)] is a very odd expression. What exactly is that supposed to do?

With a symbolic (unassigned) a < 0 does not evaluate, and therefore it is not True, i.e. TrueQ[a < 0] evaluates to False. PatternTest only matches on an explicit True, therefore _?(a < 0 &) will match nothing until a has a value. Since the pattern represents an empty set a is free of it.

I see that you fundamentally changed the question.

As noted in the first part of my answer PatternTest matches only on an explicitly True return of its test function. Since a < 0 does not evaluate it does not match.

  • $\begingroup$ So how can I make it return a condition on a? $\endgroup$ – MaPo Jan 27 '17 at 15:21
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    $\begingroup$ @MaPo FreeQ, like most (all?) *Q functions returns either True or False. It is not the correct tool to return a conditional expression. Have you looked at Reduce, Resolve, etc.? $\endgroup$ – Mr.Wizard Jan 27 '17 at 15:21

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