Deleting digits
  In 2000, J. Shallit introduced a special partial ordering of a subset of positive integers and proposed the problem of finding the set of minimal elements with respect to this ordering. Shallit himself solved this problem for the set of primes and also for the set of composite numbers. In this recreational mathematics note, we compute the minimal sets of a few other arithmetically interesting sets and discuss questions on size and shape of minimal sets in general. 
Ioulia N. Baoulina, Martin Kreh, J\"orn Steuding
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