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Pairing 1 to 22 with differences of 1 or 11

Question

In how many ways can the set {1,2,…,22}\{1,2,\dots,22\} be split into 1111 two-element blocks so that in every block the two numbers differ by exactly 11 or exactly 1111?

Short answer

145145. Write 11–1111 above 1212–2222 in a 2×112\times11 grid: differences of 1111 are vertical neighbours and differences of 11 are horizontal neighbours, plus one extra link between 1111 and 1212. Domino tilings of a 2×n2\times n strip follow the Fibonacci recursion, giving 144144 for n=11n=11. Pairing {11,12}\{11,12\} forces everything else, adding 11 more way.

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