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 Pi

 http://functions.wolfram.com/02.03.06.0013.01

 Input Form

 Pi == 2 Sum[(2 k - 1)!!/((2 k + 1) (2 k)!!), {k, 0, Infinity}]

 Standard Form

 Cell[BoxData[RowBox[List["\[Pi]", "\[Equal]", RowBox[List["2", RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["k", "=", "0"]], "\[Infinity]"], FractionBox[RowBox[List[RowBox[List["(", RowBox[List[RowBox[List["2", "k"]], "-", "1"]], ")"]], "!!"]], RowBox[List[RowBox[List["(", RowBox[List[RowBox[List["2", "k"]], "+", "1"]], ")"]], RowBox[List[RowBox[List["(", RowBox[List["2", "k"]], ")"]], "!!"]]]]]]]]]]]]]

 MathML Form

 π 2 k = 0 ( 2 k - 1 ) !! ( 2 k + 1 ) ( 2 k ) !! 2 k 0 Factorial2 2 k -1 2 k 1 Factorial2 2 k -1 [/itex]

 Rule Form

 Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", "\[Pi]", "]"]], "\[RuleDelayed]", RowBox[List["2", " ", RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["k", "=", "0"]], "\[Infinity]"], FractionBox[RowBox[List[RowBox[List["(", RowBox[List[RowBox[List["2", " ", "k"]], "-", "1"]], ")"]], "!!"]], RowBox[List[RowBox[List["(", RowBox[List[RowBox[List["2", " ", "k"]], "+", "1"]], ")"]], " ", RowBox[List[RowBox[List["(", RowBox[List["2", " ", "k"]], ")"]], "!!"]]]]]]]]]]]]]

 Date Added to functions.wolfram.com (modification date)

 2001-10-29