Allow Christiansen-Hu in 2D

FIAT/christiansen_hu.py

@@ -70,8 +70,6 @@ class ChristiansenHu(finite_element.CiarletElement):
     """The Christiansen-Hu C^0(Worsey-Farin) linear macroelement with divergence in P0.
     This element belongs to a Stokes complex, and is paired with unsplit DG0."""
     def __init__(self, ref_el, degree=1):
-        if ref_el.get_shape() != TETRAHEDRON:
-            raise ValueError("Christiansen-Hu only defined on tetrahedra")
         if degree != 1:
             raise ValueError("Christiansen-Hu only defined for degree = 1")
         poly_set = ChristiansenHuSpace(ref_el, degree)

test/unit/test_fiat.py

@@ -319,6 +319,7 @@ def __init__(self, a, b):
     "AlfeldSorokina(T)",
     "AlfeldSorokina(S)",
     "ArnoldQin(T)",
+    "ChristiansenHu(T)",
     "ChristiansenHu(S)",
     "GuzmanNeilan(T)",
     "GuzmanNeilan(S)",

test/unit/test_macro.py

@@ -383,7 +383,7 @@ def test_AlfeldSorokinaSpace(cell, degree):
     assert numpy.allclose(residual, 0)
 
 
-@pytest.mark.parametrize("family", ("CH", "GN"))
+@pytest.mark.parametrize("family", ("AQ", "CH", "GN"))
 def test_minimal_stokes_space(cell, family):
     # Test that the C0 Stokes space is spanned by a C0 basis
     # Also test that its divergence is constant
@@ -396,12 +396,14 @@ def test_minimal_stokes_space(cell, family):
     if family == "GN":
         degree = sd
         space = ExtendedGuzmanNeilanSpace
-    elif sd == 2:
-        degree = 2
-        space = ArnoldQinSpace
-    elif sd == 3:
+    elif family == "CH":
         degree = 1
         space = ChristiansenHuSpace
+    elif family == "AQ":
+        if sd != 2:
+            return
+        degree = 2
+        space = ArnoldQinSpace
 
     W = space(cell, degree)
     V = space(cell, degree, reduced=True)