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acosh
Inverse hyperbolic cosine.SYNOPSIS:
double x, y, acosh();
y = acosh(x);
DESCRIPTION:
Returns the inverse hyperbolic cosine of an argument.
If 1 <= x < 1.5, a rational approximation:
sqrt(z) * P(z)/Q(z)
where z = x-1, is used. Otherwise:
acosh(x) = log(x + sqrt((x-1)(x+1)).
ACCURACY:
Relative error:
arithmetic domain # trials peak rms
DEC 1,3 30000 4.2e-17 1.1e-17
IEEE 1,3 30000 4.6e-16 8.7e-17
ERROR MESSAGES:
message condition value returned
domain |x| < 1 NAN