Search: id:A046413
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%I A046413
%S A046413 3,4,5,7,11,17,47,59,71,139,211,251
%N A046413 Numbers n such that the repunit of length n (11...11, with n 1's) has 
               exactly 2 prime factors.
%C A046413 347, 457, 461 and 701 are also terms. The only other possible terms up 
               to 1000 are 263, 311, 509, 557, 617, 647 and 991; repunits of these 
               lengths are known to be composite but the linked sources do not provide 
               their factors. - Rick L. Shepherd (rshepherd2(AT)hotmail.com), Mar 
               11 2003
%C A046413 The Yousuke Koide reference now shows repunit of length 263 partially 
               factored, no longer possible candidate for this sequence. - Ray Chandler 
               (rayjchandler(AT)sbcglobal.net), Sep 06 2005
%D A046413 Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 
               60.
%H A046413 P. De Geest, <a href="http://www.worldofnumbers.com/repunits.htm">Repunits 
               prime factors</a>
%H A046413 Yousuke KOIDE, <a href="http://www.h4.dion.ne.jp/~rep">Factorizations 
               of Repunit Numbers</a>.
%H A046413 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               Repunit.html">Repunit</a>
%e A046413 a(n)=7 so 1111111 = 239*4649.
%Y A046413 Cf. A000042, A004022 (the actual primes), A046053, A102782.
%Y A046413 Sequence in context: A095880 A076497 A137950 this_sequence A120635 A113533 
               A023713
%Y A046413 Adjacent sequences: A046410 A046411 A046412 this_sequence A046414 A046415 
               A046416
%K A046413 nonn,base
%O A046413 1,1
%A A046413 Patrick De Geest (pdg(AT)worldofnumbers.com), Jul 15 1998.
%E A046413 More terms from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Mar 11 
               2003

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