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%I #8 May 13 2013 01:48:46
%S 3,4,5,6,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,
%T 89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,
%U 179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269,271,277,281
%N Numbers having exactly two divisors d such that d+1 is not a divisor.
%C A137921(a(n)) = 2;
%C a(n) = A000040(n-1) for n > 4.
%o (PARI) a(n)=if(n>4,prime(n-1),[3,4,5,6][n]) \\ _Charles R Greathouse IV_, Aug 21 2011
%K nonn,easy
%O 1,1
%A _Reinhard Zumkeller_, Feb 23 2008
%E Corrected by _Charles R Greathouse IV_, Apr 19 2010