cp's OEIS Frontend

A339309 Numbers k such that A228553(k) is twice a prime.

%I A339309 #10 Feb 06 2021 22:07:36
%S A339309 2,5,7,8,9,10,14,16,34,53,55,83,91,140,172,180,182,187,189,242,246,
%T A339309 254,265,288,361,398,404,421,435,490,508,559,574,623,706,727,736,747,
%U A339309 770,773,777,984,985,1065,1093,1106,1125,1187,1245,1256,1287,1341,1346,1361,1372,1381,1449,1467,1573,1596
%N A339309 Numbers k such that A228553(k) is twice a prime.
%H A339309 Robert Israel, <a href="/A339309/b339309.txt">Table of n, a(n) for n = 1..3016</a>
%e A339309 a(3) = 7 is a term because A228553(7) = 82 = 2*41 and 41 is prime.
%p A339309 f:= proc(n)
%p A339309   local S;
%p A339309   S:= select(t -> isprime(t) and isprime(2*n-t), [seq(i, i=3..n, 2)]);
%p A339309   add(t*(2*n-t), t=S)
%p A339309 end proc:
%p A339309 f(2):= 4:
%p A339309 filter:= proc(t) local v; v:= f(t); v::even and isprime(v) end proc:
%p A339309 select(filter, [$1..2000]);
%Y A339309 Cf. A228553, A339307.
%K A339309 nonn
%O A339309 1,1
%A A339309 _J. M. Bergot_ and _Robert Israel_, Nov 29 2020