Integer comparison

[tbennun]

Running the following code I expect the result of statement 5 to be True:

In [3]: M = sp.Symbol('M', positive=True, integer=True)

In [4]: N = sp.Symbol('N', positive=True, integer=True)

In [5]: M*N >= N
Out[5]: M*N >= N

This doesn't work as well, but ask is said to be WIP:

In [12]: sp.ask(sp.Q.nonnegative(M*N - N))

Which assumption am I missing on the symbols?

[oscarbenjamin]

This one works:

In [13]: M*(N - 1) >= 0
Out[13]: True

Probably ask should handle things like this but there isn't a general solver for systems of inequalities that it could use.

[tbennun]

That is indeed interesting (I'm guessing because N >= 1 evaluates to True). Strangely enough M*N - N >= 0 evaluates to a Relational. Also interesting:

In [22]: sp.solve((M*N - N) >= 0, M)
Out[22]: True

(N works too)

More tidbits:

In [32]: ((M*N - N) >= 0).simplify()
Out[32]: True

In [33]: (M*N >= N).simplify()
Out[33]: N <= M*N

which of course means that solving M*N >= N works as well.

[u7122029]

So I'm currently working on this issue and I think I have a solution, however, I'm having an issue with making the test function. My test function looks like

# sympy/simplify/tests/test_simplify.py
def test_issue_23518():
    M = Symbol("M", positive=True, integer=True)
    N = Symbol("N", positive=True, integer=True)

    expr = M*N >= N
    expr = expr.simplify() # (X)
    assert expr == True

Unfortunately when I try to run the simplify method on the line marked X, nothing happens. When I try using breakpoints to step into the method, the line just gets skipped like nothing happened.

On the other hand, if I make a testing file on the root directory of the project that looks like:

# testing.py
from sympy import *

M = Symbol("M", positive=True, integer=True)
N = Symbol("N", positive=True, integer=True)

expr = M*N >= N
expr = expr.simplify()
assert expr == True

The code runs fine without any assertion errors. May I please know what is going wrong here and how to remedy the situation?