For n=1, "2" contains the first 1 prime as a subsequence, so a(1)=1.
For n=2, "23" contains each of the first 2 primes as a subsequence, so a(2)=2.
For n=3, "235" contains each of the first 3 primes as a subsequence, so a(3)=3.
For n=4, "2357" contains each of the first 4 primes as a subsequence, so a(4)=4.
For n=5, we cannot add any new primes as the 5th prime (11) requires two extra digits, so a(5)=a(4)=4.
For n=6, "112357" contains each of the first 7 primes (2 to 17) as a subsequence, so a(6)=7.
For n=7, "1231579" contains each of the first 12 primes (2 to 37) as a subsequence, so a(7)=12.
