%I
%S 3,4,5,7,11,17,47,59,71,139,211,251,311,347,457,461
%N Numbers n such that the repunit of length n (11...11, with n 1's) has exactly 2 prime factors.
%C 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_, Mar 11 2003
%C The Yousuke Koide reference now shows repunit of length 263 partially factored, no longer possible candidate for this sequence.  _Ray Chandler_, Sep 06 2005
%C The term 263 has 3 prime factors, 617 has one prime factor and a large composite. For terms between 1000 to 2000, other possible terms are 1117, 1213, 1259, 1291, 1373, 1447, 1607, 1637, 1663, 1669, 1759, 1823, 1949, 1987, 2063 & 2087.  _Robert G. Wilson v_, Apr 26 2010
%C All terms are either primes or squares of primes in A004023. In particular, the only composite below a million is 4.  _Charles R Greathouse IV_, Nov 21 2014
%D Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 60.
%H P. De Geest, <a href="http://www.worldofnumbers.com/repunits.htm">Repunits prime factors</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/repunit">Factorizations of 11...11 (Repunit)</a>.
%H Yousuke Kiode, <a href="http://www.h4.dion.ne.jp/~rep">Factorizations of Repunit Numbers</a>.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Repunit.html">Repunit</a>
%e a(n)=7 so 1111111 = 239*4649.
%Y Cf. A000042, A004022 (the actual primes), A046053, A102782.
%K nonn,base
%O 1,1
%A _Patrick De Geest_, Jul 15 1998
%E More terms from _Rick L. Shepherd_, Mar 11 2003
%E a(13)a(16) from _Robert G. Wilson v_, Apr 26 2010
