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   http://functions.wolfram.com/01.06.16.0078.01
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    Sin[I n (ArcSech[z]/2)] == I Sqrt[(1 - z)/z] 
   Sum[(-1)^k Binomial[-k + n - 1, k] 2^(n/2 - k - 1) 
     ((1 + z)/z)^((n - 1)/2 - k), {k, 0, Floor[(n - 1)/2]}] /; 
 Element[n, Integers] && n > 0 
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   Cell[BoxData[RowBox[List[RowBox[List[RowBox[List["Sin", "[", RowBox[List["\[ImaginaryI]", " ", "n", " ", FractionBox[RowBox[List[" ", RowBox[List["ArcSech", "[", "z", "]"]]]], "2"]]], "]"]], "\[Equal]", RowBox[List["\[ImaginaryI]", " ", SqrtBox[FractionBox[RowBox[List["1", "-", "z"]], "z"]], " ", RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["k", "=", "0"]], RowBox[List["Floor", "[", FractionBox[RowBox[List["n", "-", "1"]], "2"], "]"]]], RowBox[List[SuperscriptBox[RowBox[List["(", RowBox[List["-", "1"]], ")"]], "k"], " ", RowBox[List["Binomial", "[", RowBox[List[RowBox[List[RowBox[List["-", "k"]], "+", "n", "-", "1"]], ",", "k"]], "]"]], " ", SuperscriptBox["2", RowBox[List[FractionBox["n", "2"], "-", "k", "-", "1"]]], " ", SuperscriptBox[RowBox[List["(", FractionBox[RowBox[List["1", "+", "z"]], "z"], ")"]], RowBox[List[FractionBox[RowBox[List["n", "-", "1"]], "2"], "-", "k"]]]]]]]]]]], "/;", RowBox[List[RowBox[List["Element", "[", RowBox[List["n", ",", "Integers"]], "]"]], "\[And]", RowBox[List["n", ">", "0"]]]]]]]] 
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   <math xmlns='http://www.w3.org/1998/Math/MathML' mathematica:form='TraditionalForm' xmlns:mathematica='http://www.wolfram.com/XML/'>  <semantics>  <mrow>  <mrow>  <mrow>  <mi> sin </mi>  <mo> ⁡ </mo>  <mo> ( </mo>  <mrow>  <mfrac>  <mrow>  <mi> ⅈ </mi>  <mo> ⁢ </mo>  <mi> n </mi>  </mrow>  <mn> 2 </mn>  </mfrac>  <mo> ⁢ </mo>  <mrow>  <msup>  <mi> sech </mi>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ( </mo>  <mi> z </mi>  <mo> ) </mo>  </mrow>  </mrow>  <mo> ) </mo>  </mrow>  <mo> ⩵ </mo>  <mrow>  <mi> ⅈ </mi>  <mo> ⁢ </mo>  <msqrt>  <mfrac>  <mrow>  <mn> 1 </mn>  <mo> - </mo>  <mi> z </mi>  </mrow>  <mi> z </mi>  </mfrac>  </msqrt>  <mo> ⁢ </mo>  <mrow>  <munderover>  <mo> ∑ </mo>  <mrow>  <mi> k </mi>  <mo> = </mo>  <mn> 0 </mn>  </mrow>  <mrow>  <mo> ⌊ </mo>  <mfrac>  <mrow>  <mi> n </mi>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  <mn> 2 </mn>  </mfrac>  <mo> ⌋ </mo>  </mrow>  </munderover>  <mrow>  <msup>  <mrow>  <mo> ( </mo>  <mrow>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  <mo> ) </mo>  </mrow>  <mi> k </mi>  </msup>  <mo> ⁢ </mo>  <semantics>  <mrow>  <mo> ( </mo>  <mtable>  <mtr>  <mtd>  <mrow>  <mi> n </mi>  <mo> - </mo>  <mi> k </mi>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </mtd>  </mtr>  <mtr>  <mtd>  <mi> k </mi>  </mtd>  </mtr>  </mtable>  <mo> ) </mo>  </mrow>  <annotation encoding='Mathematica'> TagBox[RowBox[List["(", GridBox[List[List[TagBox[RowBox[List["n", "-", "k", "-", "1"]], Identity, Rule[Editable, True]]], List[TagBox["k", Identity, Rule[Editable, True]]]]], ")"]], InterpretTemplate[Function[Binomial[Slot[1], Slot[2]]]], Rule[Editable, False]] </annotation>  </semantics>  <mo> ⁢ </mo>  <msup>  <mn> 2 </mn>  <mrow>  <mfrac>  <mi> n </mi>  <mn> 2 </mn>  </mfrac>  <mo> - </mo>  <mi> k </mi>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  </msup>  <mo> ⁢ </mo>  <msup>  <mrow>  <mo> ( </mo>  <mfrac>  <mrow>  <mi> z </mi>  <mo> + </mo>  <mn> 1 </mn>  </mrow>  <mi> z </mi>  </mfrac>  <mo> ) </mo>  </mrow>  <mrow>  <mfrac>  <mrow>  <mi> n </mi>  <mo> - </mo>  <mn> 1 </mn>  </mrow>  <mn> 2 </mn>  </mfrac>  <mo> - </mo>  <mi> k </mi>  </mrow>  </msup>  </mrow>  </mrow>  </mrow>  </mrow>  <mo> /; </mo>  <mrow>  <mi> n </mi>  <mo> ∈ </mo>  <msup>  <mi> ℕ </mi>  <mo> + </mo>  </msup>  </mrow>  </mrow>  <annotation-xml encoding='MathML-Content'>  <apply>  <ci> Condition </ci>  <apply>  <eq />  <apply>  <sin />  <apply>  <times />  <apply>  <times />  <imaginaryi />  <ci> n </ci>  <apply>  <power />  <cn type='integer'> 2 </cn>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <arcsech />  <ci> z </ci>  </apply>  </apply>  </apply>  <apply>  <times />  <imaginaryi />  <apply>  <power />  <apply>  <times />  <apply>  <plus />  <cn type='integer'> 1 </cn>  <apply>  <times />  <cn type='integer'> -1 </cn>  <ci> z </ci>  </apply>  </apply>  <apply>  <power />  <ci> z </ci>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <cn type='rational'> 1 <sep /> 2 </cn>  </apply>  <apply>  <sum />  <bvar>  <ci> k </ci>  </bvar>  <lowlimit>  <cn type='integer'> 0 </cn>  </lowlimit>  <uplimit>  <apply>  <floor />  <apply>  <times />  <apply>  <plus />  <ci> n </ci>  <cn type='integer'> -1 </cn>  </apply>  <apply>  <power />  <cn type='integer'> 2 </cn>  <cn type='integer'> -1 </cn>  </apply>  </apply>  </apply>  </uplimit>  <apply>  <times />  <apply>  <power />  <cn type='integer'> -1 </cn>  <ci> k </ci>  </apply>  <apply>  <ci> Binomial </ci>  <apply>  <plus />  <ci> n </ci>  <apply>  <times />  <cn type='integer'> -1 </cn>  <ci> k </ci>  </apply>  <cn type='integer'> -1 </cn>  </apply>  <ci> k </ci>  </apply>  <apply>  <power />  <cn type='integer'> 2 </cn>  <apply>  <plus />  <apply>  <times />  <ci> n </ci>  <apply>  <power />  <cn type='integer'> 2 </cn>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <times />  <cn type='integer'> -1 </cn>  <ci> k </ci>  </apply>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <power />  <apply>  <times />  <apply>  <plus />  <ci> z </ci>  <cn type='integer'> 1 </cn>  </apply>  <apply>  <power />  <ci> z </ci>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <plus />  <apply>  <times />  <apply>  <plus />  <ci> n </ci>  <cn type='integer'> -1 </cn>  </apply>  <apply>  <power />  <cn type='integer'> 2 </cn>  <cn type='integer'> -1 </cn>  </apply>  </apply>  <apply>  <times />  <cn type='integer'> -1 </cn>  <ci> k </ci>  </apply>  </apply>  </apply>  </apply>  </apply>  </apply>  </apply>  <apply>  <in />  <ci> n </ci>  <apply>  <ci> SuperPlus </ci>  <ci> ℕ </ci>  </apply>  </apply>  </apply>  </annotation-xml>  </semantics>  </math> 
   
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  | Cell[BoxData[RowBox[List[RowBox[List["HoldPattern", "[", RowBox[List["Sin", "[", RowBox[List[FractionBox["1", "2"], " ", "\[ImaginaryI]", " ", "n_", " ", RowBox[List["ArcSech", "[", "z_", "]"]]]], "]"]], "]"]], "\[RuleDelayed]", RowBox[List[RowBox[List["\[ImaginaryI]", " ", SqrtBox[FractionBox[RowBox[List["1", "-", "z"]], "z"]], " ", RowBox[List[UnderoverscriptBox["\[Sum]", RowBox[List["k", "=", "0"]], RowBox[List["Floor", "[", FractionBox[RowBox[List["n", "-", "1"]], "2"], "]"]]], RowBox[List[SuperscriptBox[RowBox[List["(", RowBox[List["-", "1"]], ")"]], "k"], " ", RowBox[List["Binomial", "[", RowBox[List[RowBox[List[RowBox[List["-", "k"]], "+", "n", "-", "1"]], ",", "k"]], "]"]], " ", SuperscriptBox["2", RowBox[List[FractionBox["n", "2"], "-", "k", "-", "1"]]], " ", SuperscriptBox[RowBox[List["(", FractionBox[RowBox[List["1", "+", "z"]], "z"], ")"]], RowBox[List[FractionBox[RowBox[List["n", "-", "1"]], "2"], "-", "k"]]]]]]]]], "/;", RowBox[List[RowBox[List["n", "\[Element]", "Integers"]], "&&", RowBox[List["n", ">", "0"]]]]]]]]]]  |  
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   Date Added to functions.wolfram.com (modification date)
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