code: def test_issue_11726():
      x, t = symbols("x t")
      f = symbols("f", cls=Function)
      X, T = symbols("X T", cls=Function)
  
      u = f(x, t)
      eq = u.diff(x, 2) - u.diff(t, 2)
      res = pde_separate(eq, u, [T(x), X(t)])
      assert res == [D(T(x), x, x) / T(x), D(X(t), t, t) / X(t)]
  
file_path: sympy/sympy/solvers/tests/test_pde.py
imports: from sympy.core.function import Derivative, Function
  from sympy.core.relational import Eq
  from sympy.core.symbol import Symbol, symbols
  from sympy.functions.elementary.exponential import exp, log
  from sympy.functions.elementary.trigonometric import cos, sin
  from sympy.core import S
  from sympy.solvers.pde import pde_separate, pde_separate_add, pde_separate_mul, pdsolve, classify_pde, checkpdesol
  from sympy.testing.pytest import raises
label_desc: Use image node_id 4 for calling a global function with example usage: test_issue_11726() without return types