Hello SeqFans, A is a derangement of the odd numbers B is a derangement of the even numbers B(n) is the absolute first difference |A(n) - A(n+1)| Always extend A with the smallest unused odd number so far not leading to a contradiction: A = 1, 3, 7, 13, 5, 15, 27, 9, 23, 39, 11, 31, 53, 17, ... B = 2 4 6 8 10 12 18 14 16 28 20 22 36 ... Best, É.