Merge pull request #935 from Li-Xiang-Ideal/patch-1

Update tensors.py

CHANGES.rst

@@ -8,9 +8,10 @@ New Builtins
 ++++++++++++
 
 
-* `Elements`
-* `RealAbs` and `RealSign`
-* `RealValuedNumberQ`
+* ``Elements``
+* ``LeviCivitaTensor``
+* ``RealAbs`` and ``RealSign``
+* ``RealValuedNumberQ``
 
 
 Compatibility

mathics/builtin/tensors.py

@@ -19,13 +19,18 @@
 """
 
 
+from sympy.combinatorics import Permutation
+from sympy.utilities.iterables import permutations
+
 from mathics.core.atoms import Integer, String
 from mathics.core.attributes import A_FLAT, A_ONE_IDENTITY, A_PROTECTED
 from mathics.core.builtin import BinaryOperator, Builtin
+from mathics.core.convert.python import from_python
 from mathics.core.evaluation import Evaluation
 from mathics.core.expression import Expression
 from mathics.core.list import ListExpression
 from mathics.core.symbols import Atom, Symbol, SymbolFalse, SymbolTrue
+from mathics.core.systemsymbols import SymbolRule, SymbolSparseArray
 from mathics.eval.parts import get_part
 
 
@@ -490,3 +495,46 @@ def eval(self, m, evaluation: Evaluation):
                 else:
                     result[col_index].append(item)
         return ListExpression(*[ListExpression(*row) for row in result])
+
+
+class LeviCivitaTensor(Builtin):
+    """
+    <url>:Levi-Civita tensor:https://en.wikipedia.org/wiki/Levi-Civita_symbol</url> \
+    (<url>:WMA link:https://reference.wolfram.com/language/ref/LeviCivitaTensor.html</url>)
+
+    <dl>
+      <dt>'LeviCivitaTensor[$d$]'
+      <dd>gives the $d$-dimensional Levi-Civita totally antisymmetric tensor.
+    </dl>
+
+    >> LeviCivitaTensor[3]
+     = SparseArray[Automatic, {3, 3, 3}, 0, {{1, 2, 3} -> 1, {1, 3, 2} -> -1, {2, 1, 3} -> -1, {2, 3, 1} -> 1, {3, 1, 2} -> 1, {3, 2, 1} -> -1}]
+
+    >> LeviCivitaTensor[3, List]
+     = {{{0, 0, 0}, {0, 0, 1}, {0, -1, 0}}, {{0, 0, -1}, {0, 0, 0}, {1, 0, 0}}, {{0, 1, 0}, {-1, 0, 0}, {0, 0, 0}}}
+    """
+
+    rules = {
+        "LeviCivitaTensor[d_Integer]/; Greater[d, 0]": "LeviCivitaTensor[d, SparseArray]",
+        "LeviCivitaTensor[d_Integer, List] /; Greater[d, 0]": "LeviCivitaTensor[d, SparseArray] // Normal",
+    }
+
+    summary_text = "give the Levi-Civita tensor with a given dimension"
+
+    def eval(self, d, type, evaluation: Evaluation):
+        "LeviCivitaTensor[d_Integer, type_]"
+
+        if isinstance(d, Integer) and type == SymbolSparseArray:
+            d = d.get_int_value()
+            perms = list(permutations([i for i in range(1, d + 1)]))
+            rules = [
+                Expression(
+                    SymbolRule,
+                    from_python(p),
+                    from_python(Permutation.from_sequence(p).signature()),
+                )
+                for p in perms
+            ]
+            return Expression(
+                SymbolSparseArray, from_python(rules), from_python([d] * d)
+            )