def deriv(self, n=1):
        """ Compute *n*-th derivative of ``self``.

        :param n: Number of derivatives.
        :type n: positive integer
        :returns: *n*-th derivative of ``self``.
        """
        if n==1:
            if self.order > 0:
                return PowerSeries(self.c[1:]*range(1,len(self.c)))
            else:
                return PowerSeries([0. * self.c[0]])
        elif n>1:
            return self.deriv().deriv(n-1)
        elif n==0:
            return self