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A163468 Indices k such that half of the k-th nonprime nonnegative integer is prime.

%I A163468 #12 Mar 04 2019 01:49:16
%S A163468 3,4,7,9,15,18,24,27,33,43,45,54,61,64,71,80,89,93,103,109,113,122,
%T A163468 129,139,151,157,161,168,172,179,201,208,217,220,237,241,250,261,268,
%U A163468 279,288,291,308,311,318,321,341,361,368,371,377,388,391,408,418,428,440
%N A163468 Indices k such that half of the k-th nonprime nonnegative integer is prime.
%C A163468 Here k-th nonprime = A141468(k) (the k-th nonprime nonnegative integer).
%H A163468 Harvey P. Dale, <a href="/A163468/b163468.txt">Table of n, a(n) for n = 1..1000</a>
%F A163468 a(n) = 2 + A065897(n). - _R. J. Mathar_, Aug 01 2009
%e A163468 3 is a term because the 3rd nonprime nonnegative integer is 4, and 4/2 = 2 is prime.
%e A163468 4 is a term because the 4th nonprime nonnegative integer is 6, and 6/2 = 3 is prime.
%e A163468 7 is a term because the 7th nonprime nonnegative integer is 10, and 10/2 = 5 is prime.
%p A163468 A141468 := proc(n) option remember ; if n = 1 then 0; else for a from procname(n-1)+1 do if not isprime(a) then RETURN(a) ; fi; od: fi; end: for n from 1 to 600 do npr := A141468(n) ; if type(npr,'even') then if isprime(npr/2) then printf("%d,",n) ; fi; fi; od: # _R. J. Mathar_, Aug 01 2009
%t A163468 Flatten[Position[Select[Range[600],!PrimeQ[#]&],_?(PrimeQ[#/2]&)]]+1 (* _Harvey P. Dale_, May 05 2016 *)
%Y A163468 Cf. A065897, A000040, A141468.
%K A163468 nonn,easy,less
%O A163468 1,1
%A A163468 _Juri-Stepan Gerasimov_, Jul 28 2009
%E A163468 a(8) corrected by _R. J. Mathar_, Aug 01 2009
%E A163468 Edited by _Jon E. Schoenfield_, Mar 04 2019