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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