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

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

Offset

1,1

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.

Links

Table of n, a(n) for n=1..8.

Mathematica

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

Prog

(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
(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

Crossrefs

Row 3 of A223701.

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

Sequence in context: A312843 A206523 A312844 * A329897 A365892 A312845

Adjacent sequences: A181445 A181446 A181447 * A181449 A181450 A181451

Keyword

fini,full,nonn,changed

Author

Artur Jasinski, Oct 21 2010

Status

approved