public SparseMatrix aat() {
        SparseMatrix AT = transpose();
        return aat(AT);
    }
@Test
    public void testAAT() {
        System.out.println("AAT");
        SparseMatrix c = sparse.aat();
        assertEquals(3, c.nrow());
        assertEquals(3, c.ncol());
        assertEquals(9, c.size());
        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);
            }
        }
    }