A sequence of binary numbers. The first number is 1. Thereafter each number a(n) is defined using the previous number a(n-1) in the following way : a(n)=a(n-1).b(n-1) where the dot means the concatenation operation and b(n-1) stands for the first number that is not present in a(n-1), using the usual order (1, 10, 11, 100, 101, etc.).