Add kn function as helper for modified Bessel function of the second kind and corresponding tests

pytensor/tensor/math.py

@@ -2453,6 +2453,11 @@ def kv(v, x):
     return kve(v, x) * exp(-x)
 
 
+def kn(n, x):
+    """Modified Bessel function of the second kind of integer order v."""
+    return kv(n, x)
+
+
 @scalar_elemwise
 def sigmoid(x):
     """Logistic sigmoid function (1 / (1 + exp(-x)), also known as expit or inverse logit"""
@@ -4337,6 +4342,7 @@ __all__ = [
     "i1",
     "iv",
     "ive",
+    "kn",
     "kv",
     "kve",
     "sigmoid",

tests/tensor/test_math_scipy.py

@@ -18,7 +18,7 @@ from pytensor import function, grad
 from pytensor import tensor as pt
 from pytensor.compile.mode import get_default_mode
 from pytensor.configdefaults import config
-from pytensor.tensor import gammaincc, inplace, kv, kve, vector
+from pytensor.tensor import gammaincc, inplace, kn, kv, kve, vector
 from tests import unittest_tools as utt
 from tests.tensor.utils import (
     _good_broadcast_unary_chi2sf,
@@ -1220,3 +1220,17 @@ def test_kv():
         out.eval({v: test_v, x: test_x}),
         scipy.special.kv(test_v[:, None], test_x[None, :]),
     )
+
+
+def test_kn():
+    n = vector("n")
+    x = vector("x")
+
+    out = kn(n[:, None], x[None, :])
+    test_n = np.array([-3, 4, 0, 5], dtype=n.type.dtype)
+    test_x = np.linspace(0, 512, 10, dtype=x.type.dtype)
+
+    np.testing.assert_allclose(
+        out.eval({n: test_n, x: test_x}),
+        scipy.special.kn(test_n[:, None], test_x[None, :]),
+    )