Hello SeqFans, a(1)=1 a(n) is the smallest integer not yet in S and not stopping S, such that a(n-1) + [the leftmost digit of a(n)] is prime. S = 1,2,3,4,7,6,5,8,9,20,30,10,11,21,22,31,... S is not a permutation of the integers > 0 (as 113, for instance, cannot be followed by anything; so 113 is forbidden in S) Best, É.