Originální popis anglicky: 
acosh, acoshf, acoshl - inverse hyperbolic cosine function
Návod, kniha: Linux Programmer's Manual
#include <math.h>
 
double acosh(double x);
 
float acoshf(float x);
 
long double acoshl(long double x);
 
Link with -lm.
The 
acosh() function calculates the inverse hyperbolic cosine of
  
x; that is the value whose hyperbolic cosine is 
x. If 
x
  is less than 1.0, 
acosh() returns not-a-number (NaN) and 
errno
  is set.
  - EDOM
- x is out of range.
SVID 3, POSIX, BSD 4.3, ISO 9899. The float and long double variants are C99
  requirements.
asinh(3), 
atanh(3), 
cosh(3), 
sinh(3),
  
tanh(3)