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A106127 Numbers k such that k-th semiprime == 2 (mod k). 0

%I

%S 1,2,4,39,51,52,71,6920,613377,613381,613405,613433,613437,613449,

%T 613455,613536,613537,613548,613557,613569,613581,613583,613587,

%U 613588,613608,613613,58155550,58155570,6384425447,6384425465,6384425505,6384425531,6384425567

%N Numbers k such that k-th semiprime == 2 (mod k).

%C No more terms through 160788536. - _Ryan Propper_, May 09 2006

%C a(34) > 10^12. - _Lucas A. Brown_, Oct 17 2020

%e a(3)=4 is a term because the 4th semiprime (i.e., 10) == 2 (mod 4).

%t SemiprimeQ[n_] := (Plus @@ Last /@ FactorInteger[n] == 2); i = 0; Do[If[SemiprimeQ[n], i++; If[Mod[n, i] == 2, Print[i]]], {n, 10^9}] (* _Ryan Propper_, May 09 2006 *)

%Y Cf. A001358.

%K hard,nonn

%O 1,2

%A _Shyam Sunder Gupta_, May 07 2005

%E More terms from _Ryan Propper_, May 09 2006

%E a(1), a(2), and a(29)-a(33) from _Lucas A. Brown_, Oct 17 2020

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Last modified October 15 21:38 EDT 2021. Contains 348034 sequences. (Running on oeis4.)