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 ArcTan

 http://functions.wolfram.com/01.15.03.0006.01

 Input Form

 ArcTan[x, 0] == (1 - UnitStep[x]) Pi /; Element[x, Reals] && x != 0

 Standard Form

 Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["ArcTan", "[", RowBox[List["x", ",", "0"]], "]"]], "\[Equal]", RowBox[List[RowBox[List["(", RowBox[List["1", "-", RowBox[List["UnitStep", "[", "x", "]"]]]], ")"]], "\[Pi]"]]]], "/;", RowBox[List[RowBox[List["x", "\[Element]", "Reals"]], "\[And]", RowBox[List["x", "\[NotEqual]", "0"]]]]]]]]

 MathML Form

 tan - 1 ( x , 0 ) ( 1 - θ UnitStep ( x ) ) π /; x TagBox["\[DoubleStruckCapitalR]", Function[List[], Reals]] x 0 Condition x 0 1 -1 UnitStep x x x 0 [/itex]

 Rule Form

 Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["ArcTan", "[", RowBox[List["x_", ",", "0"]], "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[RowBox[List["(", RowBox[List["1", "-", RowBox[List["UnitStep", "[", "x", "]"]]]], ")"]], " ", "\[Pi]"]], "/;", RowBox[List[RowBox[List["x", "\[Element]", "Reals"]], "&&", RowBox[List["x", "\[NotEqual]", "0"]]]]]]]]]]

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

 2007-05-02