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%I #15 Oct 29 2024 15:41:07
%S 25,35,55,65,77,85,91,115,121,133,143,155,175,185,217,221,235,245,247,
%T 253,275,287,295,301,305,323,325,343,361,365,371,377,395,407,413,415,
%U 445,455,475,485,493,497,511,517,527,533,545,553,583,595,655,665,685
%N Odd composite numbers k such that 2*k-3 is prime.
%H Jianing Song, <a href="/A348423/b348423.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = (A348421(n) + 3)/2.
%e 25 is a term since 25 is an odd composite number and 2*25 - 3 = 47 is prime.
%t Select[Range[1, 700, 2], CompositeQ[#] && PrimeQ[2*# - 3] &] (* _Amiram Eldar_, Oct 18 2021 *)
%o (PARI) isA348423(n) = !isprime(n) && (n%2) && isprime(2*n-3)
%Y Cf. A002145, A348421.
%K nonn,easy
%O 1,1
%A _Jianing Song_, Oct 18 2021