# std::atanh(std::complex)

< cpp‎ | numeric‎ | complex

C++
 Language Headers Library concepts Language support library Diagnostics library Utilities library Strings library Containers library Algorithms library Iterators library Numerics library Input/output library Localizations library Regular expressions library (C++11) Atomic operations library (C++11) Thread support library (C++11) Filesystem library (C++17) Technical Specifications

Numerics library
 Common mathematical functions Special mathematical functions Floating-point environment (C++11) Complex numbers Numeric arrays Pseudo-random number generation Compile-time rational arithmetic (C++11) Numeric algorithms gcd(C++17) lcm(C++17) Generic numeric operations iota(C++11) accumulate inner_product adjacent_difference partial_sum

std::complex
Member functions
Non-member functions
 real imag abs arg norm conj proj(C++11) polar operator""ioperator""ifoperator""il(C++14)(C++14)(C++14)
Exponential functions
Power functions
Trigonometric functions
 asin(C++11) acos(C++11) atan(C++11)
Hyperbolic functions
 asinh(C++11) acosh(C++11) atanh(C++11)

 Defined in header  template< class T > complex atanh( const complex& z ); (since C++11)

Computes the complex arc hyperbolic tangent of z with branch cuts outside the interval [−1; +1] along the real axis.

## Contents

### Parameters

 z - complex value

### Return value

If no errors occur, the complex arc hyperbolic tangent of z is returned, in the range of a half-strip mathematically unbounded along the real axis and in the interval [−iπ/2; +iπ/2] along the imaginary axis.

### Error handling and special values

Errors are reported consistent with math_errhandling

If the implementation supports IEEE floating-point arithmetic,

• std::atanh(std::conj(z)) == std::conj(std::atanh(z))
• std::atanh(-z) == -std::atanh(z)
• If z is (+0,+0), the result is (+0,+0)
• If z is (+0,NaN), the result is (+0,NaN)
• If z is (+1,+0), the result is (+∞,+0) and FE_DIVBYZERO is raised
• If z is (x,+∞) (for any finite positive x), the result is (+0,π/2)
• If z is (x,NaN) (for any finite nonzero x), the result is (NaN,NaN) and FE_INVALID may be raised
• If z is (+∞,y) (for any finite positive y), the result is (+0,π/2)
• If z is (+∞,+∞), the result is (+0,π/2)
• If z is (+∞,NaN), the result is (+0,NaN)
• If z is (NaN,y) (for any finite y), the result is (NaN,NaN) and FE_INVALID may be raised
• If z is (NaN,+∞), the result is (±0,π/2) (the sign of the real part is unspecified)
• If z is (NaN,NaN), the result is (NaN,NaN)

### Notes

Although the C++ standard names this function "complex arc hyperbolic tangent", the inverse functions of the hyperbolic functions are the area functions. Their argument is the area of a hyperbolic sector, not an arc. The correct name is "complex inverse hyperbolic tangent", and, less common, "complex area hyperbolic tangent".

Inverse hyperbolic tangent is a multivalued function and requires a branch cut on the complex plane. The branch cut is conventionally placed at the line segmentd (-∞,-1] and [+1,+∞) of the real axis.

The mathematical definition of the principal value of the inverse hyperbolic tangent is atanh z =
 ln(1+z)-ln(1-z) 2
.

For any z, atanh(z) =
 atan(iz) i

### Example

#include <iostream>
#include <complex>

int main()
{
std::cout << std::fixed;
std::complex<double> z1(2, 0);
std::cout << "atanh" << z1 << " = " << std::atanh(z1) << '\n';

std::complex<double> z2(2, -0.0);
std::cout << "atanh" << z2 << " (the other side of the cut) = "
<< std::atanh(z2) << '\n';

// for any z, atanh(z) = atanh(iz)/i
std::complex<double> z3(1,2);
std::complex<double> i(0,1);
std::cout << "atanh" << z3 << " = " << std::atanh(z3) << '\n'
<< "atan" << z3*i << "/i = " << std::atan(z3*i)/i << '\n';
}

Output:

atanh(2.000000,0.000000) = (0.549306,1.570796)
atanh(2.000000,-0.000000) (the other side of the cut) = (0.549306,-1.570796)
atanh(1.000000,2.000000) = (0.173287,1.178097)
atan(-2.000000,1.000000)/i = (0.173287,1.178097)

### See also

 asinh(std::complex)(C++11) computes area hyperbolic sine of a complex number (function template)  acosh(std::complex)(C++11) computes area hyperbolic cosine of a complex number (function template)  tanh(std::complex) computes hyperbolic tangent of a complex number (function template)  atanh(C++11) computes the inverse hyperbolic tangent (artanh(x)) (function)  C documentation for catanh