public BigMatrix aat() {
        BigMatrix C = new BigMatrix(m, m);
        C.mm(NO_TRANSPOSE, this, TRANSPOSE, this);
        C.uplo(LOWER);
        return C;
    }
@Test
    public void testAAT() {
        System.out.println("AAT");
        BigMatrix c = matrix.aat();
        assertEquals(c.nrow(), 3);
        assertEquals(c.ncol(), 3);
        for (int i = 0; i < C.length; i++) {
            for (int j = 0; j < C[i].length; j++) {
                assertEquals(C[i][j], c.get(i, j), 1E-7);
            }
        }
    }