man:acosh(3)

ACOSH(3) Linux Programmer's Manual ACOSH(3) NAME acosh, acoshf, acoshl — inverse hyperbolic cosine function SYNOPSIS #include <math.h> double acosh(double x); float acoshf(float x); long double acoshl(long double x); Link with -lm. DESCRIPTION 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. ERRORS EDOM x is out of range. CONFORMING TO SVr4, POSIX.1-2001, 4.3BSD, C89. The float and long double variants are C99 requirements. SEE ALSO asinh(3), atanh(3), cacosh(3), cosh(3), sinh(3), tanh(3) 2002-07-25 ACOSH(3)