def cosh(x, err=defaultError):
"""
cosh(x [,err]) returns the hyperbolic cosine of x.
"""
if err <= defaultError:
series = ExponentialPowerSeries(
itertools.cycle((
rational.Integer(1),
0,
)))
rx = rational.Rational(x)
if rx == 0:
return rational.Integer(1)
return series(rx, err)
else:
return rational.Rational(math.cosh(x))
