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A131867 a(n) is the 2^n-th semiprime.

%I A131867 #34 Mar 02 2019 02:20:24
%S A131867 4,6,10,22,46,93,202,407,849,1774,3693,7671,15999,33146,68703,142682,
%T A131867 295003,610757,1261573,2603453,5369633,11058907,22758881,46796443,
%U A131867 96132103,197329777,404737537,829538129,1698995201,3477431507,7113030933,14540737711
%N A131867 a(n) is the 2^n-th semiprime.
%C A131867 The PARI code allows one to resume at the k-th semiprime, e.g., SP(295003,65536) and to change the output interval, e.g., SP(_,_,10) = A114125, SP(_,_,-1) = A001358.
%H A131867 Dana Jacobsen, <a href="/A131867/b131867.txt">Table of n, a(n) for n = 0..59</a> (first 45 terms from Robert G. Wilson v)
%F A131867 a(n) = A001358(2^n).
%e A131867 a(0)=4 is the first semiprime;
%e A131867 a(1)=6 is the 2nd semiprime;
%e A131867 a(16)=295003 is the 65536th semiprime.
%o A131867 (PARI) SP( n=0 /*tested number*/,c=0 /*count of semiprimes*/, step=2)={ local( l=c+!c ); /* negative/positive step means arithmetic/geometric progression of output threshold l */ until( 0, until(l<=c++,until(bigomega(n+=1)==2,));print1(/*c ":" */ n ", "); if(step>0,l*=step,l-=step))}
%o A131867 (Perl) use ntheory ":all"; my($i,$g)=(0,0); forsemiprimes { print $g++," $_\n" if ++$i == 1<<$g; } 10**8; # _Dana Jacobsen_, Sep 10 2018
%o A131867 (Perl) use ntheory ":all"; print "$_ ",nth_semiprime(1<<$_),"\n" for 0..40; # _Dana Jacobsen_, Oct 08 2018
%Y A131867 Cf. A001358 (semiprimes), A114125.
%K A131867 nonn
%O A131867 0,1
%A A131867 _M. F. Hasler_, Oct 04 2007
%E A131867 a(23)-a(28) from _Donovan Johnson_, Nov 11 2008
%E A131867 a(29)-a(33) from _Max Alekseyev_, May 07 2010