import math


def H_approx(n):
    """
    Returns an approximate value of n-th harmonic number
    http://en.wikipedia.org/wiki/Harmonic-number
    """
    # Euler-Mascheroni constant
    gamma = 0.57721566490153286060651209008240243104215933593992
    return gamma + math.log(n) + 0.5/n - 1./(12*n**2) + 1./(120*n**4)