fix: allow Theta on Abs when positivity unknown

src/estimates/order_of_magnitude.py

@@ -116,7 +116,13 @@ def __new__(cls, expr):
         if isinstance(expr, OrderOfMagnitude):
             return expr
 
-        if not expr.is_positive:
+        # Abs(x) is nonnegative but sympy often leaves .is_positive as None (zero possible).
+        # Still a valid order of magnitude when not identically zero.
+        if expr.is_positive:
+            pass
+        elif expr.is_nonnegative is True and expr.is_zero is not True:
+            pass
+        else:
             print(f"Warning: a non-positive argument {expr!s} was passed to Theta.")
             return Undefined()
 

tests/test_all.py

@@ -110,3 +110,12 @@ def test_subst_all_solution_reversed(self, capsys):
     def test_sympy_simplify_solution(self, capsys):
         sympy_simplify_solution()
         self.proof_complete(capsys)
+
+    def test_theta_abs_nonnegative(self):
+        """Theta(Abs(n)) must not become Undefined when positivity is unknown."""
+        from sympy import Abs, Symbol
+        from estimates.order_of_magnitude import Theta, Undefined
+        n = Symbol("n", integer=True)
+        t = Theta(Abs(n))
+        assert not isinstance(t, Undefined)
+        assert str(t).startswith("Theta")