| Step | Statement | Justification |
|---|---|---|
| 1 | N₁ | is a palindrome with an even number of digitsBy construction: A‖B‖B‖A |
| 2 | N₁ ≡ 0 (mod 11) | Even-digit palindrome ⇒ divisible by 11 |
| 3 | N₂ | By construction: X‖Y‖Y‖X |
| 4 | N₂ ≡ 0 (mod 11) | Even-digit palindrome ⇒ divisible by 11 |
| 5 | N₁ − N₂ ≡ 0 − 0 ≡ 0 (mod 11) | Congruence subtraction |