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 A242341 Numbers k such that k*10^k - 1 is a semiprime. 1
 1, 6, 20, 29, 35, 40, 79, 164, 185, 198, 201, 218, 248, 249, 251, 264, 267, 274, 305, 323, 339, 344, 350, 362, 432, 539 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The semiprimes of this form are: 9, 5999999, 1999999999999999999999, 2899999999999999999999999999999, ... From Robert Israel, Sep 04 2016: (Start) k == 1 (mod 3) is in the sequence iff (k*10^k-1)/3 is prime. The sequence includes 185, 198, 201, 251, 267, 274, and 1795. (End) a(27) >= 596. Below 1000, 785 and 833 are in the sequence. Unknown factorization for 596, 669, 917, 933. - Hugo Pfoertner, Jul 29 2019 LINKS MAPLE issemiprime:= proc(n) local F, t;     F:= ifactors(n, easy)[2];     t:= add(f[2], f=F);     if t = 1 then        if type(F[1][1], integer) then return false fi     elif t = 2 then        return not hastype(F, name)     else # t > 2        return false     fi;     F:= ifactors(n)[2];     return evalb(add(f[2], f=F)=2); end proc: select(t -> issemiprime(t*10^t-1), [\$1..80]); # Robert Israel, Sep 04 2016 MATHEMATICA Select[Range[70], PrimeOmega[# 10^# - 1]==2&] PROG (MAGMA) IsSemiprime:=func; [n: n in [2..70] | IsSemiprime(s) where s is n*10^n-1]; (PARI) is(n)=bigomega(n*10^n-1)==2 \\ Charles R Greathouse IV, Sep 04 2016 CROSSREFS Cf. similar sequences listed in A242273. Cf. A059671, A064756. Sequence in context: A064771 A006036 A308710 * A140738 A325593 A226363 Adjacent sequences:  A242338 A242339 A242340 * A242342 A242343 A242344 KEYWORD nonn,more,hard AUTHOR Vincenzo Librandi, May 12 2014 EXTENSIONS Terms 1 and 79 from Robert Israel, Sep 04 2016 a(8)-a(26) from Hugo Pfoertner, Jul 29 2019 STATUS approved

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Last modified February 23 13:30 EST 2020. Contains 332159 sequences. (Running on oeis4.)