catan, catanf,
  catanl —
complex arc tangent functions
#include <complex.h>
double complex
  
  catan(double
    complex z);
float complex
  
  catanf(float
    complex z);
long double complex
  
  catanl(long
    double complex z);
These functions compute the complex arc tangent of z, with
  branch cuts outside the interval [- i, +i] along the imaginary axis.
These functions return the complex arc tangent value, in the range of a strip
  mathematically unbounded along the imaginary axis and in the interval
  [-pi/2, +pi/2] along the real axis.
ctan(3), IEEE Std
  1003.1-2001 (“POSIX.1”)
  ⟨complex.h⟩
Portions of this text are reprinted and reproduced in electronic form from IEEE
  Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
  Operating System Interface (POSIX), The Open Group Base Specifications Issue
  6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
  Engineers, Inc and The Open Group. In the event of any discrepancy between
  this version and the original IEEE and The Open Group Standard, the original
  IEEE and The Open Group Standard is the referee document. The original
  Standard can be obtained online at http://www.opengroup.org/unix/online.html .