public double kurtosis() {
		double kurtosis = NaN;
		if (_n > 3L) {
			final double var = _m2.value()/(_n - 1);
			kurtosis = _n <= 3L || var < 10E-20
				? 0.0
				: (_n*(_n + 1.0)*_m4.value() -
					3.0*_m2.value()*_m2.value()*(_n - 1.0))/
					((_n - 1.0)*(_n - 2.0)*(_n - 3.0)*var*var);
		}
		return kurtosis;
	}
Return the kurtosis of values recorded, or {@code Double.NaN} if less
than four values have been recorded.

@see <a href="https://en.wikipedia.org/wiki/Kurtosis">Kurtosis</a>

@return the kurtosis of values, or {@code NaN} if less than four values
        have been recorded
jenetics/src/main/java/io/jenetics/stat/MomentStatistics.java