A181447 Numbers k such that 3 is the largest prime factor of k^2 - 1.

2, 5, 7, 17

Offset

1,1

Comments

Sequence is finite and complete, for proof see A175607.

Search for terms can be restricted to the range from 2 to A175607(2) = 17; primepi(3) = 2.

Links

Table of n, a(n) for n=1..4.

Mathematica

Select[Range[20], FactorInteger[#^2-1][[-1, 1]]==3&]

Prog

(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
(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

Crossrefs

Cf. A175607, A181448-A181470, A181568.

Sequence in context: A322440 A067580 A325210 * A082088 A075881 A208241

Adjacent sequences: A181444 A181445 A181446 * A181448 A181449 A181450

Keyword

nonn,fini,full

Author

Artur Jasinski, Oct 21 2010

Status

approved