feat: add a test.

src/mrinufft/operators/base.py

@@ -9,7 +9,7 @@
 from functools import partial
 import warnings
 import numpy as np
-from mrinufft._utils import _power_method
+from mrinufft._utils import power_method
 
 from mrinufft.density import get_density
 
@@ -204,7 +204,7 @@ def get_lipschitz_cst(self, max_iter=10, **kwargs):
         tmp_op = self.__class__(
             self.samples, self.shape, density=self.density, n_coils=1, **kwargs
         )
-        return _power_method(max_iter, tmp_op)
+        return power_method(max_iter, tmp_op)
 
     @property
     def uses_sense(self):

tests/test_interfaces.py

@@ -97,3 +97,22 @@ def test_interfaces_autoadjoint(operator):
         reldiff[i] = abs(rightadjoint - leftadjoint) / abs(leftadjoint)
     print(reldiff)
     assert np.mean(reldiff) < 5e-5
+
+
+def test_interface_lipschitz(operator):
+    """Test the Lipschitz constant of the operator."""
+    spec_rad = operator.get_lipschitz_cst(20)
+
+    def AHA(x):
+        return operator.adj_op(operator.op(x))
+
+    L = np.zeros(10)
+    for i in range(10):
+        img_data = image_from_op(operator)
+        img2_data = image_from_op(operator)
+
+        L[i] = np.linalg.norm(AHA(img2_data) - AHA(img_data)) / np.linalg.norm(
+            img2_data - img_data
+        )
+
+    assert np.mean(L) < 1.1 * spec_rad