Merge pull request #936 from gchq/fix/incorrect-gradient-periodic-ker…

…nel-test

fix: correct sign of periodic kernel expected gradient

CHANGELOG.md

@@ -48,6 +48,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Pylint pre-commit hook is now configured as the Pylint docs recommend. (https://github.com/gchq/coreax/pull/899)
 - Type annotations so that core coreax package passes Pyright. (https://github.com/gchq/coreax/pull/906)
 - Type annotations so that the example scripts pass Pyright. (https://github.com/gchq/coreax/pull/921)
+- Incorrectly-implemented tests for the gradients of `PeriodicKernel`. (https://github.com/gchq/coreax/pull/936)
 
 ### Changed
 

tests/unit/test_kernels.py

@@ -1971,7 +1971,7 @@ def problem(self, request, kernel: PeriodicKernel) -> _Problem:  # noqa: C901
         )
         return _Problem(x, y, expected_distances, modified_kernel)
 
-    def expected_grad_x(
+    def expected_grad_y(
         self, x: ArrayLike, y: ArrayLike, kernel: PeriodicKernel
     ) -> np.ndarray:
         x = np.atleast_2d(x)
@@ -1998,10 +1998,10 @@ def expected_grad_x(
                 )
         return expected_gradients
 
-    def expected_grad_y(
+    def expected_grad_x(
         self, x: ArrayLike, y: ArrayLike, kernel: PeriodicKernel
     ) -> np.ndarray:
-        return -self.expected_grad_x(x, y, kernel)
+        return -self.expected_grad_y(x, y, kernel)
 
     def expected_divergence_x_grad_y(
         self, x: ArrayLike, y: ArrayLike, kernel: PeriodicKernel