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 cl:functions:asin [2019/06/15 02:00] cl:functions:asin [2019/06/18 09:00] (current) Line 1: Line 1: + ====== Function ASIN, ACOS, ATAN ====== + + ====Syntax==== + * **asin** //number// → //​radians// ​ + * **acos** //number// → //​radians// ​ + * **atan** //number1 ''&​optional''​ number2// → //radians// + + ====Arguments and Values==== + * //number// - a //​[[CL:​Glossary:​number]]//​. + * //number1// - a //​[[CL:​Glossary:​number]]//​ if //number2// is not supplied, or a //​[[CL:​Glossary:​real]]//​ if //number2// is supplied. + * //number2// - a //​[[CL:​Glossary:​real]]//​. + * //radians// - a //​[[CL:​Glossary:​number]]//​ (of radians). + + ====Description==== + + **asin**, **acos**, and **atan** compute the arc sine, arc cosine, and arc tangent respectively. + + The arc sine, arc cosine, and arc tangent (with only //number1// supplied) functions can be defined mathematically for //number// or //number1// specified as //x// as in the below table. + + ^ Function ​   ^ Definition ​                                          ^ + | Arc sine    | ''​-i * log(i * //x// + sqrt(1 - //​x//<​sup>​2​))'' ​            | + | Arc cosine ​ | ''​(π / 2) - arcsin(//​x//​)'' ​                                      | + | Arc tangent | ''​-i * log((1 + i * //x//) * sqrt(1 / (1 + //​x//<​sup>​2​)))''​ | + + These formulae are mathematically correct, assuming completely accurate computation. They are not necessarily the simplest ones for real-valued computations. + + If both //number1// and //number2// are supplied for **atan**, the result is the arc tangent of //​number1//<​nowiki>///​number2//​. The value of **atan** is always between ''​-π''​ (exclusive) and ''​π''​ (inclusive) when minus zero is not supported. The range of the two-argument arc tangent when minus zero is supported includes ''​-π''​. + + For a //​[[CL:​Glossary:​real]]//​ //​number1//,​ the result is a //​[[CL:​Glossary:​real]]//​ and lies between ''​-π/​2''​ and~''​π/​2''​ (both exclusive). //number1// can be a //​[[CL:​Glossary:​complex]]//​ if //number2// is not supplied. If both are supplied, //number2// can be zero provided //number1// is not zero. + + The following definition for arc sine determines the range and branch cuts: + + <​blockquote>​ + arcsin(//​z//​) = -i * log(i * //z// + sqrt(1 - //​z//<​sup>​2​)) + ​ + + The branch cut for the arc sine function is in two pieces: one along the negative real axis to the left of~''​-1''​ (inclusive),​ continuous with quadrant II, and one along the positive real axis to the right of~''​1''​ (inclusive),​ continuous with quadrant IV. The range is that strip of the complex plane containing numbers whose real part is between ''​-π/​2''​ and~''​π/​2''​. A number with real part equal to ''​-π/​2''​ is in the range if and only if its imaginary part is non-negative;​ a number with real part equal to ''​π/​2''​ is in the range if and only if its imaginary part is non-positive. + + The following equivalent definitions for arc cosine determines the range and branch cuts: + + <​blockquote>​ + //​arccos//​(//​z//​) = π / 2 - arcsin(//​z//​) + + //​arccos//​(//​z//​) = -i * log(z + i 8 sqrt(1 - z<​sup>​2​)) + + //​arccos//​(//​z//​) = (2 * log(sqrt((1 + //z//) / 2) + i * sqrt((1 - //z//) / 2))) / i + ​ + + The branch cut for the arc cosine function is in two pieces: one along the negative real axis to the left of ''​-1''​ (inclusive),​ continuous with quadrant II, and one along the positive real axis to the right of ''​1''​ (inclusive),​ continuous with quadrant IV. This is the same branch cut as for arc sine. The range is that strip of the complex plane containing numbers whose real part is between 0 and~''​π''​. A number with real part equal to 0 is in the range if and only if its imaginary part is non-negative;​ a number with real part equal to ''​π''​ is in the range if and only if its imaginary part is non-positive. + + The following definition for (one-argument) arc tangent determines the range and branch cuts: + + <​blockquote>​ + arctan(//​z//​) = (log(1 + i * z) - log(1 - i * z)) / 2 * i + ​ + + Beware of simplifying this formula; "​obvious"​ simplifications are likely to alter the branch cuts or the values on the branch cuts incorrectly. The branch cut for the arc tangent function is in two pieces: one along the positive imaginary axis above ''​i''​ (exclusive),​ continuous with quadrant II, and one along the negative imaginary axis below ''​-i''​ (exclusive),​ continuous with quadrant IV. The points ''​i''​ and ''​-i''​ are excluded from the domain. The range is that strip of the complex plane containing numbers whose real part is between ''​-π/​2''​ and ''​π/​2''​. A number with real part equal to ''​-π/​2''​ is in the range if and only if its imaginary part is strictly positive; a number with real part equal to ''​π/​2''​ is in the range if and only if its imaginary part is strictly negative. Thus the range of arc tangent is identical to that of arc sine with the points ''​-π/​2''​ and ''​π/​2''​ excluded. + + For **atan**, the signs of //number1// (indicated as ''​x''​) and //number2// (indicated as ''​y''​) are used to derive quadrant information. The next table details various special cases. + + The asterisk (*) indicates that the entry in the figure applies to implementations that support minus zero. + + ^   ^ ''​y''​ Condition ​ ^ ''​x''​ Condition ​ ^ Cartesian locus ^ Range of result ​          ^ + |   | ''​y = 0'' ​       | ''​x > 0'' ​       | Positive x-axis |''​ 0'' ​                    | + | * | ''​y = +0'' ​      | ''​x > 0'' ​       | Positive x-axis |''​+0'' ​                    | + | * | ''​y = -0'' ​      | ''​x > 0'' ​       | Positive x-axis |''​-0'' ​                    | + |   | ''​y > 0'' ​       | ''​x > 0'' ​       | Quadrant I      |''​0 < //result// < π/​2'' ​  | + |   | ''​y > 0'' ​       | ''​x = 0'' ​       | Positive y-axis |''​π/​2'' ​                   | + |   | ''​y > 0'' ​       | ''​x < 0'' ​       | Quadrant II     ​|''​π/​2 < //result// < π'' ​  | + |   | ''​y = 0'' ​       | ''​x < 0'' ​       | Negative x-axis |''​ π'' ​                    | + | * | ''​y = +0'' ​      | ''​x < 0'' ​       | Negative x-axis |''​+π'' ​                    | + | * | ''​y = -0'' ​      | ''​x < 0'' ​       | Negative x-axis |''​-π'' ​                    | + |   | ''​y < 0'' ​       | ''​x < 0'' ​       | Quadrant III    |''​-π < //result// < -π/​2''​ | + |   | ''​y < 0'' ​       | ''​x = 0'' ​       | Negative y-axis |''​-π/​2'' ​                  | + |   | ''​y < 0'' ​       | ''​x > 0'' ​       | Quadrant IV     ​|''​-π/​2 < //result// < 0'' ​ | + |   | ''​y = 0'' ​       | ''​x = 0'' ​       | Origin ​         | undefined consequences ​   | + | * | ''​y = +0'' ​      | ''​x = +0'' ​      | Origin ​         |''​+0'' ​                    | + | * | ''​y = -0'' ​      | ''​x = +0'' ​      | Origin ​         |''​-0'' ​                    | + | * | ''​y = +0'' ​      | ''​x = -0'' ​      | Origin ​         |''​+π'' ​                    | + | * | ''​y = -0'' ​      | ''​x = -0'' ​      | Origin ​         |''​-π'' ​                    | + + ====Examples==== + <​blockquote>​ + (asin 0) <​r>​0.0 + (acos #c(0 1)) <​r>#​C(1.5707963267948966 -0.8813735870195432) + (/ (atan 1 (sqrt 3)) 6) <​r>​0.087266 + (atan #c(0 2)) <​r>#​C(-1.5707964 0.54930615) + ​ + + ====Affected By==== + None. + + ====Exceptional Situations==== + **acos** and **asin** should signal an //​[[CL:​Glossary:​error]]//​ of type **[[CL:​Types:​type-error]]** if //number// is a //​[[CL:​Glossary:​number]]//​. **atan** should signal **[[CL:​Types:​type-error]]** if one argument is supplied and that argument is not a //​[[CL:​Glossary:​number]]//,​ or if two arguments are supplied and both of those arguments are not //​[[CL:​Glossary:​reals]]//​. + + **acos**, **asin**, and **atan** might signal **[[CL:​Types:​arithmetic-error]]**. + + ====See Also==== + * **[[CL:​Functions:​log|Function LOG]]** + * **[[CL:​Functions:​sqrt|Function SQRT]]** + * {\secref\FloatSubstitutability} + + ====Notes==== + The result of either **asin** or **acos** can be a //​[[CL:​Glossary:​complex]]//​ even if //number// is not a //​[[CL:​Glossary:​complex]]//;​ this occurs when the absolute value of //number// is greater than one. + + \issue{COMPLEX-ATANH-BOGUS-FORMULA:​TWEAK-MORE} \issue{COMPLEX-ATAN-BRANCH-CUT:​TWEAK} \issue{REAL-NUMBER-TYPE:​X3J13-MAR-89} \issue{REAL-NUMBER-TYPE:​X3J13-MAR-89} \issue{COMPLEX-ATANH-BOGUS-FORMULA:​TWEAK-MORE} \issue{COMPLEX-ATAN-BRANCH-CUT:​TWEAK} \issue{IEEE-ATAN-BRANCH-CUT:​SPLIT} \issue{COMPLEX-ATAN-BRANCH-CUT:​TWEAK}

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