csqrt, csqrtf,
  csqrtl —
complex square root functions
#include <complex.h>
double complex
  
  csqrt(double
    complex z);
float complex
  
  csqrtf(float
    complex z);
long double complex
  
  csqrtl(long
    double complex z);
These functions compute the complex square root of z, with
  a branch cut along the negative real axis.
These functions return the complex square root value, in the range of the right
  half-plane (including the imaginary axis).
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 .