1. A Paper by Shallit on Primes
We consider here the positive integers with respect to their unique decimal
expansions, where each n ∈ N is given by n = Pk
j=0 αj10j for some non-
negative integer k and digit sequence αkαk−1 . . . α0. With slight abuse of
notation, we also use n to denote αkαk−1 . . . α0. For such sequences of digits
(as well as for the numbers represented by the corresponding expansions) we
write x ⊳ y if x is a subsequence of y, which means that either x = y or x can
be obtained from y by deleting some digits of y. For example, 514 ⊳ 352148.
The main problem is as follows: given a set S ⊂ N, find the smallest possible
set M ⊂ S such that for all s ∈ S there exists m ∈ M with m⊳ s. As a matter
of fact, the set