1999 OIM Problems/Problem 4

Problem

Let $B$ be an integer greater than 10 such that each of its digits belongs to the set {1, 3, 7, 9}. Show that $B$ has a prime factor greater than or equal to 11.

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also

https://www.oma.org.ar/enunciados/ibe14.htm