# std::asin(std::complex)

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< cpp‎ | numeric‎ | complex

Defined in header `<complex>` |  |
template< class T > std::complex <T> asin( const std::complex <T>& z ); |  | (since C++11)

Computes complex arc sine of a complex value z. Branch cut exists outside the interval [−1, +1] along the real axis.

### Parameters

z | - | complex value

### Return value

If no errors occur, complex arc sine of z is returned, in the range of a strip unbounded along the imaginary axis and in the interval [−π/2, +π/2] along the real axis.

Errors and special cases are handled as if the operation is implemented by `-i * std::asinh(i * z)`, where `i` is the imaginary unit.

### Notes

Inverse sine (or arc sine) is a multivalued function and requires a branch cut on the complex plane. The branch cut is conventionally placed at the line segments (-∞,-1) and (1,∞) of the real axis.

The mathematical definition of the principal value of arc sine is \(\small \arcsin z = -{\rm i}\ln({\rm i}z+\sqrt{1-z^2})\)arcsin z = -iln(iz + √1-z2
).

For any z, \(\small{ \arcsin(z) = \arccos(-z) - \frac{\pi}{2} }\)asin(z) = acos(-z) -

π
2

.

### Example

Run this code

    #include <cmath>
    #include <complex>
    #include <iostream>

    int main()
    {
        std::cout << std::fixed;
        std::complex<double> z1(-2.0, 0.0);
        std::cout << "asin" << z1 << " = " << std::asin(z1) << '\n';

        std::complex<double> z2(-2.0, -0.0);
        std::cout << "asin" << z2 << " (the other side of the cut) = "
                  << std::asin(z2) << '\n';

        // for any z, asin(z) = acos(−z) − pi / 2
        const double pi = std::acos(-1);
        std::complex<double> z3 = std::acos(z2) - pi / 2;
        std::cout << "sin(acos" << z2 << " - pi / 2) = " << std::sin(z3) << '\n';
    }

Output:

    asin(-2.000000,0.000000) = (-1.570796,1.316958)
    asin(-2.000000,-0.000000) (the other side of the cut) = (-1.570796,-1.316958)
    sin(acos(-2.000000,-0.000000) - pi / 2) = (2.000000,0.000000)

### See also

acos(std::complex) (C++11) | computes arc cosine of a complex number (\({\small\arccos{z}}\)arccos(z)) (function template)
atan(std::complex) (C++11) | computes arc tangent of a complex number (\({\small\arctan{z}}\)arctan(z)) (function template)
sin(std::complex) | computes sine of a complex number (\({\small\sin{z}}\)sin(z)) (function template)
asinasinfasinl (C++11)(C++11) | computes arc sine (\({\small\arcsin{x}}\)arcsin(x)) (function)
asin(std::valarray) | applies the function std::asin to each element of valarray (function template)
C documentation for casin

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