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%I #22 Nov 26 2022 16:56:30
%S 2,5,7,17
%N Numbers k such that 3 is the largest prime factor of k^2 - 1.
%C Sequence is finite and complete, for proof see A175607.
%C Search for terms can be restricted to the range from 2 to A175607(2) = 17; primepi(3) = 2.
%t Select[Range[20], FactorInteger[#^2-1][[-1, 1]]==3&]
%o (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 (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 Cf. A175607, A181448-A181470, A181568.
%K nonn,fini,full
%O 1,1
%A _Artur Jasinski_, Oct 21 2010