Incomplete simplify

[proy87]

simplify(2*((2*x - 3*y**2)*(3*x - (3*x**2 - 2*y)/(2*x - 3*y**2)) + (3*x**2 - 2*y)*(3*y*(3*x**2 - 2*y)/(2*x - 3*y**2) - 1))/(2*x - 3*y**2)**2) returns 2*x*y*(27*x**3 - 54*x*y + 27*y**3 + 8)/(8*x**3 - 36*x**2*y**2 + 54*x*y**4 - 27*y**6).
But 8*x**3 - 36*x**2*y**2 + 54*x*y**4 - 27*y**6 is (2*x - 3*y**2)**3.

[mostlyaman]

As mentioned in the sympy docs at https://docs.sympy.org/latest/tutorial/simplification.html

But simplify() has a pitfall. It just applies all the major simplification operations in SymPy, and uses heuristics to determine the simplest result. But “simplest” is not a well-defined term.

To get specific simplifications, you can use other simplification methods that specialise for your use. In this case, factor returns a factorised numerator and denominator.

>>> factor(2*((2*x - 3*y**2)*(3*x - (3*x**2 - 2*y)/(2*x - 3*y**2)) + (3*x**2 - 2*y)*(3*y*(3*x**2 - 2*y)/(2*x - 3*y**2) - 1))/(2*x - 3*y**2)**2)
2*x*y*(3*x + 3*y + 2)*(9*x**2 - 9*x*y - 6*x + 9*y**2 - 6*y + 4)/(2*x - 3*y**2)**3

factor does the job but if you are suggesting that simplify should output factorised denominator, then a change may be required.