FEAT: simplify Hankel polynomial internally

src/ampform/dynamics/form_factor.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from functools import lru_cache
 from typing import Any
 
 import sympy as sp
@@ -37,12 +38,16 @@ class BlattWeisskopfSquared(sp.Expr):
     _latex_repr_ = R"B_{{{angular_momentum}}}^2\left({z}\right)"
 
     def evaluate(self) -> sp.Expr:
-        z, angular_momentum = self.args
-        return (
-            sp.Abs(SphericalHankel1(angular_momentum, 1)) ** 2
-            / sp.Abs(SphericalHankel1(angular_momentum, sp.sqrt(z))) ** 2
+        ell = self.angular_momentum
+        z = sp.Dummy("z", nonnegative=True, real=True)
+        expr = (
+            sp.Abs(SphericalHankel1(ell, 1)) ** 2
+            / sp.Abs(SphericalHankel1(ell, sp.sqrt(z))) ** 2
             / z
         )
+        if not ell.free_symbols:
+            expr = expr.doit().simplify()
+        return expr.xreplace({z: self.z})
 
 
 @unevaluated(implement_doit=False)
@@ -75,10 +80,29 @@ def evaluate(self) -> sp.Expr:
         return (
             (-sp.I) ** (1 + l)  # type:ignore[operator]
             * (sp.exp(z * sp.I) / z)
-            * sp.Sum(
+            * _SymbolicSum(
                 sp.factorial(l + k)
                 / (sp.factorial(l - k) * sp.factorial(k))
                 * (sp.I / (2 * z)) ** k,  # type:ignore[operator]
                 (k, 0, l),
             )
         )
+
+
+class _SymbolicSum(sp.Sum):
+    """See [TR-029](https://compwa.github.io/report/029.html) for why this class is needed."""
+
+    def doit(self, deep: bool = True, **kwargs) -> sp.Expr:
+        if _get_indices(self):
+            expression = self.args[0]
+            indices = self.args[1:]
+            return _SymbolicSum(expression.doit(deep=deep, **kwargs), *indices)
+        return super().doit(deep=deep, **kwargs)
+
+
+@lru_cache(maxsize=None)
+def _get_indices(expr: sp.Sum) -> set[sp.Basic]:
+    free_symbols = set()
+    for index in expr.args[1:]:
+        free_symbols.update(index.free_symbols)
+    return {s for s in free_symbols if not isinstance(s, sp.Dummy)}

tests/dynamics/test_dynamics.py

@@ -21,32 +21,14 @@
 
 
 class TestBlattWeisskopfSquared:
-    def test_max_angular_momentum(self):
+    def test_factorials(self):
         z = sp.Symbol("z")
         angular_momentum = sp.Symbol("L", integer=True)
         form_factor = BlattWeisskopfSquared(z, angular_momentum)
         form_factor_9 = form_factor.subs(angular_momentum, 8).evaluate()
         factor, z_power, _ = form_factor_9.args
         assert factor == 4392846440677
         assert z_power == z**8
-        assert BlattWeisskopfSquared.max_angular_momentum is None
-        BlattWeisskopfSquared.max_angular_momentum = 1
-        assert form_factor.evaluate() == sp.Piecewise(
-            (1, sp.Eq(angular_momentum, 0)),
-            (2 * z / (z + 1), sp.Eq(angular_momentum, 1)),
-        )
-        BlattWeisskopfSquared.max_angular_momentum = None
-
-    def test_unevaluated_expression(self):
-        z = sp.Symbol("z")
-        ff1 = BlattWeisskopfSquared(z, angular_momentum=1)
-        ff2 = BlattWeisskopfSquared(z, angular_momentum=2)
-        assert ff1.max_angular_momentum is None
-        assert ff2.max_angular_momentum is None
-        BlattWeisskopfSquared.max_angular_momentum = 3
-        assert ff1.max_angular_momentum is 3  # noqa: F632
-        assert ff2.max_angular_momentum is 3  # noqa: F632
-        BlattWeisskopfSquared.max_angular_momentum = None
 
 
 class TestEnergyDependentWidth: