0!! == 1
Cell[BoxData[RowBox[List[RowBox[List["0", "!!"]], "\[Equal]", "1"]]]]
<math xmlns='http://www.w3.org/1998/Math/MathML' mathematica:form='TraditionalForm' xmlns:mathematica='http://www.wolfram.com/XML/'> <semantics> <mrow> <mrow> <mn> 0 </mn> <mo> !! </mo> </mrow> <mo> ⩵ </mo> <mn> 1 </mn> </mrow> <annotation-xml encoding='MathML-Content'> <apply> <eq /> <apply> <ci> Factorial2 </ci> <cn type='integer'> 0 </cn> </apply> <cn type='integer'> 1 </cn> </apply> </annotation-xml> </semantics> </math>
Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["0", "!!"]], "]"]], "\[RuleDelayed]", "1"]]]]
2001-10-29