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I am trying to simplify some expressions using assumptions and got an inconsistent behavior of Mathematica 9 when working with indexed variables. Consider 

Assuming[Subscript[x,_] ∈ Reals, Refine[Im[Subscript[x, 10]]]]

0

Assuming[Subscript[x,_] ∈ Reals && Subscript[x,_] > 0, Refine[Sign[Subscript[x,10]]]]

Sign[Subscript[x, 10]]

Assuming[Subscript[x,10] > 0, Refine[Sign[Subscript[x,10]]]]

1

It appears that assumptions about an indexed variable being real work, but assumptions about the positivity of an indexed variable seem not to work.

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  • $\begingroup$ Have you tried Assuming[x \[Element] Reals, Refine[Im[x]]] Assuming[x \[Element] Reals && x > 0, Refine[Sign[x]]] Assuming[x > 0, Refine[Sign[x]]]? $\endgroup$
    – Dr. belisarius
    Sep 11, 2013 at 16:36
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    $\begingroup$ This works as expected. My problem is that assumptions on patterns can only work for domains. $\endgroup$
    – yarchik
    Sep 12, 2013 at 8:41

1 Answer 1

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It is not Assuming that supports general patterns, but Element. This explains why 

Assuming[x > 0, Refine@Sign[x]]

gives 1, while

Assuming[x[_] > 0, Refine@Sign[ x[1] ]]

does not.

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    $\begingroup$ Just a note: seems like Element only supports patterns with domains such as Reals, not regions such as Interval. Not directly relevant to this question, but in case someone finds this later on... $\endgroup$
    – Chris K
    Apr 11, 2019 at 15:43
  • $\begingroup$ @ChrisK That's not obvious at all, but reading under Details on the Element doc page makes it clear why it's so. $\endgroup$
    – Szabolcs
    Apr 11, 2019 at 19:09

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