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 PrimePi

 http://functions.wolfram.com/13.04.27.0001.01

 Input Form

 PrimePi[x] == Sum[1, {i, 1, n}] /; Inequality[Prime[n], LessEqual, x, Less, Prime[n + 1]]

 Standard Form

 Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["PrimePi", "[", "x", "]"]], "\[Equal]", RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["i", "=", "1"]], "n"], "1"]]]], "/;", RowBox[List[RowBox[List["Prime", "[", "n", "]"]], "\[LessEqual]", "x", "<", RowBox[List["Prime", "[", RowBox[List["n", "+", "1"]], "]"]]]]]]]]

 MathML Form

 π TagBox["\[Pi]", PrimePi] ( x ) i = 1 n 1 /; prime ( n ) x < prime ( n + 1 ) Condition PrimePi x i 1 n 1 Inequality prime n x prime n 1 [/itex]

 Rule Form

 Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["PrimePi", "[", "x_", "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["i", "=", "1"]], "n"], "1"]], "/;", RowBox[List[RowBox[List["Prime", "[", "n", "]"]], "\[LessEqual]", "x", "<", RowBox[List["Prime", "[", RowBox[List["n", "+", "1"]], "]"]]]]]]]]]]

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

 2001-10-29