Covariance between observed values.

        s and t are one domain of the integral (i.e. the integral between s and t)
        sprime and tprime are another domain of the integral (i.e. the integral between sprime and tprime)

        We're interested in how correlated these two integrals are.

        Note: We've not multiplied by the variance, this is done in K.

def k_xx(self,t,tprime,s,sprime,l):
        """Covariance between observed values.

        s and t are one domain of the integral (i.e. the integral between s and t)
        sprime and tprime are another domain of the integral (i.e. the integral between sprime and tprime)

        We're interested in how correlated these two integrals are.

        Note: We've not multiplied by the variance, this is done in K."""
        return 0.5 * (l**2) * ( self.g((t-sprime)/l) + self.g((tprime-s)/l) - self.g((t - tprime)/l) - self.g((s-sprime)/l))