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 A180640 #11 Oct 29 2022 10:20:28 %S A180640 2,3,5,11,13,23,29,37,43,47,59,61,67,73,83,103,107,113,131,137,139, %T A180640 149,157,167,173,179,193,211,223,227,229,233,263,269,277,281,283,293, %U A180640 311,313,317,331,347,353,359,367,373,383,389,397,409,421,439,443,457,467 %N A180640 Primes P such that P < (largest prime factor of (P-1)) * (largest prime factor of (P+1)). %e A180640 For n = 3, a(3)=11. %e A180640 The prime P = 11 %e A180640 P-1 = 10 (largest prime factor of 10 is 5) %e A180640 P+1 = 12 (largest prime factor of 12 is 3) %e A180640 11 < 5*3. %t A180640 Select[Prime[Range[100]],#<(FactorInteger[#-1][[-1,1]] FactorInteger[#+1][[-1,1]])&] (* _Harvey P. Dale_, Feb 22 2011 *) %o A180640 (PARI) isok(p) = (p==2) || (isprime(p) && (p < vecmax(factor(p-1)[, 1]) * vecmax(factor(p+1)[, 1]))); \\ _Michel Marcus_, Oct 29 2022 %Y A180640 Cf. A180641. See also A103666, A103667. %K A180640 nonn %O A180640 1,1 %A A180640 _Karl Hovekamp_, Sep 14 2010 %E A180640 Initial term, i.e., 2, added by _Harvey P. Dale_, Feb 22 2011