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%I #17 Jun 30 2021 09:09:09
%S 1,3,5,7,11,19,27,43,75,115,171,307,555,691,1383,2467,3627,7243,10995,
%T 17587,28973,57943,87963,136627,267573,465293,807629,1407093,2390667,
%U 4216531,7122293,11245133,21756267,39652589
%N a(n) is the smallest positive integer that cannot be obtained by using the number 2 at most n times and the operators +, -, *, /.
%H G. Bannay, <a href="https://web.archive.org/web/20061201125224/http://gilles.bannay.free.fr/jeux_us.html">Countdown Problem</a>
%H Austin T. Mohr, <a href="http://austinmohr.com/work/A071997.py">Recursive Python implementation</a>
%e a(3)=5 because using 2 at most thrice we can get 2/2=1, 2=2, 2+(2/2)=3, 2+2=4, but we cannot obtain 5 in the same way.
%Y Cf. A048175.
%K nonn,hard,more
%O 1,2
%A Koksal Karakus (karakusk(AT)hotmail.com), Jun 17 2002
%E a(18)-a(34) from _Austin T. Mohr_, May 17 2021
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