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Reduce only generically correct?

Mathematica Asked by Gunther Reissig on March 23, 2021

While it is clearly said in the Mathematica documentation that functions like FullSimplify yield only ‘generically correct’ results, I have found no mention of such restrictions for Reduce. However, consider the following two examples (Input):

Gamma[x] [Element] Reals

Gamma[0] [Element] Reals

Reduce[ForAll[x, x [Element] Reals, Gamma[x] [Element] Reals], x, Reals]

The output (Version 12.0.0) will be:

Gamma[x] [Element] Reals

False

True

of which the last output is incorrect. The second example is even more problematic, as Mathematica tells us that Not[x!=0 || y!=0] is a contradiction (Input):

expr = Implies[x == y, 1/x - 1/y == 0]

Reduce[expr, {x, y}, Reals]

% // Simplify

Reduce[Not[expr], {x, y}, Reals]

Output is:

x == y [Implies] 1/x - 1/y == 0

x < 0 || (x == 0 && (y < 0 || y > 0)) || x > 0

x != 0 || y != 0

False

Questions:

Is there a way to always get correct answers to queries such as those in the two examples above (using Reduce or any other Mathematica commands)? If not, is there a way to be always notified whenever the result is only ‘generically correct’?

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