A181447 - OEIS

%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