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

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

Offset: 1

Author

Artur Jasinski, Oct 21 2010

Keywords

• fini
• full
• nonn

Comments

Numbers k such that A076605(k) = 7.

Sequence is finite and complete, for proof see A175607.

Search for terms can be restricted to the range from 2 to A175607(4) = 8749; primepi(7) = 4.

Crossrefs

Row 4 of A223701.

Cf. A076605, A175607, A181447, A181448, A181450-A181470, A181568.

Programs

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

Mathematica

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

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