We proceed as follows for one fixed $ i $:
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- eq d_i $ and $ d_{i+2}
- eq d_{i+1} $, provided those indices exist.
- Letâs fix a position $ i $, and count the number of sequences where the repeated pair is at $ i, i+1 $, and no other consecutive digits are equal.
- Choose the repeated digit: 3 choices.
- For the digit at position $ i-1 $ (if it exists), it must be different from the repeated digit: 2 choices.
- For the digit at position $ i+2 $ (if it exists), it must be different from the repeated digit: 2 choices.
