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A242205 Numbers n such that n*5^n+1 is semiprime. 1
1, 2, 4, 16, 21, 40, 76, 113, 153, 288 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
The semiprimes of this form are 6, 51, 2501, 2441406250001, 10013580322265626, 363797880709171295166015625001, ...
a(11) >= 1006. - Hugo Pfoertner, Aug 05 2019
LINKS
MATHEMATICA
Select[Range[90], PrimeOmega[# 5^# + 1] == 2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..90] | IsSemiprime(s) where s is n*5^n+1];
CROSSREFS
Cf. similar sequences listed in A242203.
Cf. A050916.
Sequence in context: A102545 A045521 A338766 * A076434 A192150 A049441
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, May 10 2014
EXTENSIONS
a(8)-a(10) from Luke March, Aug 13 2015
STATUS
approved

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Last modified March 28 10:55 EDT 2024. Contains 371241 sequences. (Running on oeis4.)