n ≡ ... [4]         |  0 |  1 |  2 |  3 | 
2n ≡ ... [4]        |  0 |  2 |  0 |  2 | 
n² + 1 ≡ ... [4]    |  1 |  2 |  1 |  2 | 
2n(n² + 1) ≡ ... [4]|  0 |  0 |  0 |  0 |