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A252658 Numbers m such that 5^m - m is a semiprime. 2
1, 3, 8, 14, 18, 48, 52, 54, 62, 72, 96, 98, 114, 186, 486, 524, 712 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

There are no other known terms under 1000 - the only possible terms are 812 and 908. - Carl Schildkraut, Aug 21 2015

LINKS

Table of n, a(n) for n=1..17.

EXAMPLE

1 is in this sequence because 5^1-1 = 2*2 is semiprime.

18 is in this sequence because 5^18-18 = 199*19169332993 and these two factors are prime.

MATHEMATICA

Select[Range[100], PrimeOmega[5^# - #]==2 &]

PROG

(MAGMA) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [m: m in [1..100] | IsSemiprime(s) where s is 5^m-m];

(PARI) isok(n)=bigomega(5^n - n)==2 \\ Anders Hellström, Aug 21 2015

CROSSREFS

Cf. similar sequences listed in A252656.

Sequence in context: A063218 A104656 A179993 * A140492 A184517 A028252

Adjacent sequences:  A252655 A252656 A252657 * A252659 A252660 A252661

KEYWORD

nonn,more

AUTHOR

Vincenzo Librandi, Dec 20 2014

EXTENSIONS

a(13)-a(17) from Carl Schildkraut, Aug 21 2015

STATUS

approved

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Last modified May 13 01:32 EDT 2021. Contains 343829 sequences. (Running on oeis4.)