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Numbers k such that A228553(k) is twice a prime.
2

%I #10 Feb 06 2021 22:07:36

%S 2,5,7,8,9,10,14,16,34,53,55,83,91,140,172,180,182,187,189,242,246,

%T 254,265,288,361,398,404,421,435,490,508,559,574,623,706,727,736,747,

%U 770,773,777,984,985,1065,1093,1106,1125,1187,1245,1256,1287,1341,1346,1361,1372,1381,1449,1467,1573,1596

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

%H Robert Israel, <a href="/A339309/b339309.txt">Table of n, a(n) for n = 1..3016</a>

%e a(3) = 7 is a term because A228553(7) = 82 = 2*41 and 41 is prime.

%p f:= proc(n)

%p local S;

%p S:= select(t -> isprime(t) and isprime(2*n-t), [seq(i, i=3..n, 2)]);

%p add(t*(2*n-t), t=S)

%p end proc:

%p f(2):= 4:

%p filter:= proc(t) local v; v:= f(t); v::even and isprime(v) end proc:

%p select(filter, [$1..2000]);

%Y Cf. A228553, A339307.

%K nonn

%O 1,1

%A _J. M. Bergot_ and _Robert Israel_, Nov 29 2020