A181448 Numbers k such that 5 is the largest prime factor of k^2 - 1.

4, 9, 11, 19, 26, 31, 49, 161

Offset: 1

Author

Artur Jasinski, Oct 21 2010

Keywords

• fini
• full
• nonn

Comments

Numbers k such that A076605(k) = 3.

Sequence is finite and complete, for proof see A175607.

Search for terms can be restricted to the range from 2 to A175607(3) = 161; primepi(5) = 3.

Crossrefs

Row 3 of A223701.

Cf. A076605, A175607, A181447, A181449-A181470, A181568.

Programs

Magma

[ n: n in [2..200] | m eq 5 where m is D[#D] where D is PrimeDivisors(n^2-1) ]; // Klaus Brockhaus, Feb 17 2011

Mathematica

Select[Range[200], FactorInteger[#^2-1][[-1, 1]]==5&]

PARI

is(n)=n=n^2-1; n>>=valuation(n,2); n/=3^valuation(n,3); n>1 && 5^valuation(n, 5)==n \\ Charles R Greathouse IV, Jul 01 2013