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A181449 Numbers n such that 7 is the largest prime factor of n^2 - 1. 2

%I

%S 6,8,13,15,29,41,55,71,97,99,127,244,251,449,4801,8749

%N Numbers n such that 7 is the largest prime factor of n^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(4) = 8749; primepi(7) = 4.

%t Select[Range[9000], FactorInteger[#^2-1][[-1, 1]]==7&]

%o (MAGMA) [ n: n in [2..9000] | m eq 7 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; forprime(p=2, 5, n/=p^valuation(n, p)); n>1 && 7^valuation(n, 7)==n \\ _Charles R Greathouse IV_, Jul 01 2013

%Y Cf. A175607, A181447, A181448, A181450-A181470, A181568.

%K fini,full,nonn

%O 1,1

%A _Artur Jasinski_, Oct 21 2010

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Last modified October 22 10:24 EDT 2019. Contains 328317 sequences. (Running on oeis4.)