... 33111 322 24 → 331 322 2411 → 331 324 221 1 → 3431 322 211 → 3431 322 211 ∈ [4]10 (1 12) the electronic journal of combinatorics 9 (2) (20 02) , #R3 (7) 15 Conversely, starting with 3431 322 211 ∈ [4]10 (1 12) , there ... 121 }, {1 12, 122 }, {1 12, 21 1}, {1 12, 21 2}, {1 12, 22 1}, { 121 , 21 2} Theorem The pairs {111, 1 12} and {111, 121 } are Wilf equivalent, and F111, 121 (x, y) = F111,1 12 (x, y) = n k (−1)n−i f111,1 12 (n, ... f1 12, 121 (n; k) = f1 12, 121 (n; k − 1) + nf1 12, 121 (n − 1, k − 1) + f1 12, 121 (n − j; k − 1), j =2 which means that f1 12, 121 (n; k) = f1 12, 121 (n − 1; k) + f1 12, 121 (n; k − 1) + (n − 1)f1 12, 121 ...