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 A181447 #22 Nov 26 2022 16:56:30 %S A181447 2,5,7,17 %N A181447 Numbers k such that 3 is the largest prime factor of k^2 - 1. %C A181447 Sequence is finite and complete, for proof see A175607. %C A181447 Search for terms can be restricted to the range from 2 to A175607(2) = 17; primepi(3) = 2. %t A181447 Select[Range[20], FactorInteger[#^2-1][[-1, 1]]==3&] %o A181447 (Magma) [ n: n in [2..20] | m eq 3 where m is D[#D] where D is PrimeDivisors(n^2-1) ]; // _Klaus Brockhaus_, Feb 17 2011 %o A181447 (PARI) is(n)=n=n^2-1;my(o=valuation(n,3)); o && n>>valuation(n/3^o,2)==1 \\ _Charles R Greathouse IV_, Jul 01 2013 %Y A181447 Cf. A175607, A181448-A181470, A181568. %K A181447 nonn,fini,full %O A181447 1,1 %A A181447 _Artur Jasinski_, Oct 21 2010