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%I #12 Jul 22 2017 09:16:15
%S 2,3,4,4,5,7,8,9,8,9,11,13,11,15,16,14,14,18,20,21
%N Number of numerical semigroups S of genus 3^n having a minimal generating set of cardinality 2.
%H Shalom Eliahou and Jorge Ramírez Alfonsín, <a href="http://www.math.univ-montp2.fr/~ramirez/semigroups.pdf">Two-generator numerical semigroups and Fermat and Mersenne numbers</a>, SIAM J. Discrete Math., 25(2), 2011, 622-630.
%F a(n) = n + 1 if 2*3^n - 1 is prime.
%F a(n) ~ n + 1.
%Y Cf. A003307, A079363.
%K nonn,more
%O 1,1
%A _Arkadiusz Wesolowski_, Jun 06 2013