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A181448 Numbers n such that 5 is the largest prime factor of n^2 - 1. 2
4, 9, 11, 19, 26, 31, 49, 161 (list; graph; refs; listen; history; text; internal format)
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(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

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

Sequence in context: A312843 A206523 A312844 * A312845 A312846 A182244

Adjacent sequences:  A181445 A181446 A181447 * A181449 A181450 A181451

KEYWORD

fini,full,nonn

AUTHOR

Artur Jasinski, Oct 21 2010

STATUS

approved

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Last modified October 16 20:55 EDT 2019. Contains 328103 sequences. (Running on oeis4.)