Generate the matrix A in the Bartlett decomposition

          A is a lower triangular matrix, with

              A(i, j) ~ sqrt of Chisq(df - i + 1) when i == j
                      ~ Normal()                  when i > j

def _genA(self):
        """
        Generate the matrix A in the Bartlett decomposition

          A is a lower triangular matrix, with

              A(i, j) ~ sqrt of Chisq(df - i + 1) when i == j
                      ~ Normal()                  when i > j
        """
        p, df = self._p, self.df
        A = np.zeros((p, p))

        for i in range(p):
            A[i, i] = sqrt(st.chi2.rvs(df - i))

        for j in range(p-1):
            for i in range(j+1, p):
                A[i, j] = np.random.randn()

        return A