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 EulerPhi

 http://functions.wolfram.com/13.06.27.0003.01

 Input Form

 PrimePi[x] == Sum[Floor[EulerPhi[k]/(k - 1)], {k, 2, Floor[x]}] /; x >= 0

 Standard Form

 Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["PrimePi", "[", "x", "]"]], "\[Equal]", RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["k", "=", "2"]], RowBox[List["Floor", "[", "x", "]"]]], RowBox[List["Floor", "[", FractionBox[RowBox[List["EulerPhi", "[", "k", "]"]], RowBox[List["k", "-", "1"]]], "]"]]]]]], "/;", RowBox[List["x", "\[GreaterEqual]", "0"]]]]]]

 MathML Form

 π TagBox["\[Pi]", PrimePi] ( x ) k = 2 x ϕ TagBox["\[Phi]", EulerPhi] ( k ) k - 1 /; x 0 Condition PrimePi x k 2 x EulerPhi k k -1 -1 x 0 [/itex]

 Rule Form

 Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["PrimePi", "[", "x_", "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["k", "=", "2"]], RowBox[List["Floor", "[", "x", "]"]]], RowBox[List["Floor", "[", FractionBox[RowBox[List["EulerPhi", "[", "k", "]"]], RowBox[List["k", "-", "1"]]], "]"]]]], "/;", RowBox[List["x", "\[GreaterEqual]", "0"]]]]]]]]

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

 2001-10-29