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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 factordb.com, Status of 1006*5^1006+1. MATHEMATICA Select[Range[90], PrimeOmega[# 5^# + 1] == 2 &] PROG (MAGMA) IsSemiprime:=func; [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: A212202 A102545 A045521 * A076434 A192150 A049441 Adjacent sequences:  A242202 A242203 A242204 * A242206 A242207 A242208 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 December 10 23:29 EST 2019. Contains 329910 sequences. (Running on oeis4.)