This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A379445 #23 Jan 22 2025 09:57:28
%S A379445 4,6,6,15,21,6,15,33,35,10,57,35,77,69,39,145,155,187,21,111,65,287,
%T A379445 55,21,85,221,159,33,133,14,143,391,161,185,95,1027,123,581,1247,445,
%U A379445 65,57,291,77,55,371,259,2147,437,377,85,55,35,86,1441,335,85,3197,329,3337
%N A379445 a(n) = gpf(prime(n)-1)*gpf(prime(n)+1), where gpf is A006530.
%C A379445 Observation: Even terms of A006881 not occurring in this sequence are, e.g., 22, 34, 38, 46, ..., due to the sparseness of Mersenne primes (A000668) and Fermat primes (A000215). Also missing are many multiples of 3, e.g., 3*{31, 67, 79, 83, 101, 103, 113, ...}, as a consequence of the gaps of A058383 and A268640 and the size distribution of prime factors, i.e., the rareness of smooth numbers.
%H A379445 Hugo Pfoertner, <a href="/A379445/b379445.txt">Table of n, a(n) for n = 2..10000</a>
%H A379445 Hugo Pfoertner, <a href="https://www.randomwalk.de/sequences/b379445_1e6.7z">1 million terms of A379445</a>, 7z compressed file (5 MB) (2025).
%F A379445 a(n) = A023503(n)*A023509(n). - _Michel Marcus_, Jan 21 2025
%e A379445 a(43390) = 146 because 2^19-1 = A000668(5) is the 43390th prime and the greatest prime factor of 2^19-2 is 73.
%t A379445 Table[Times @@ Map[FactorInteger[#][[-1, 1]] &, Prime[n] + {-1, 1}], {n, 2, 61}] (* _Michael De Vlieger_, Jan 20 2025 *)
%o A379445 (PARI) a379445(n) = my (p=prime(n), fm=factor(p-1), fp=factor(p+1)); fm[#fm~,1]*fp[#fp~,1]
%Y A379445 Cf. A006530, A023503, A023509, A087713.
%Y A379445 Each term > 4 is element of A006881.
%Y A379445 Cf. A000215, A000668, A058383, A268640.
%K A379445 nonn
%O A379445 2,1
%A A379445 _Hugo Pfoertner_, Dec 28 2024