Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #17 Jan 29 2019 23:59:20
%S 2,2,2,2,4,2,2,3,4,3,2,3,11,3,22,7,4,2,18,7,4,23,6,23,18,5,44,23,4,98,
%T 14,3,11,2,11,7,11,2,18,28,8,16,2,102,4,9,11,3,8,5,174,24,63,3,2,103,
%U 22,23,130,7,22,16,18,2
%N Least k > 1 such that prime(k)*2^n - 1 is prime, or zero if never prime.
%C As N increases, it appears that (Sum_{i=1..N} a(i)) / (Sum_{i=1..N} i) tends to 1/2, i.e., the partial sums grow roughly proportional to the triangular numbers.
%C It is conjectured that a(42228) is the first 0 term. This corresponds to the first Riesel number, 509203, which happens to be prime. See A101036. - _T. D. Noe_, Mar 23 2011
%H Pierre CAMI, <a href="/A187467/b187467.txt">Table of n, a(n) for n = 1..4100</a>
%F a(n) = primepi(A126715(n)). - _T. D. Noe_, Mar 10 2011
%F a(n) >= A179289(n). - _R. J. Mathar_, Mar 19 2011
%p A187467 := proc(n) local k; for k from 2 do if isprime( ithprime(k)*2^n-1) then return k; end if; end do: end proc: # _R. J. Mathar_, Mar 19 2011
%Y Cf. A101036, A126715, A187468.
%K nonn
%O 1,1
%A _Pierre CAMI_, Mar 10 2011