%I A055520
%S A055520 1,2,3,4,5,6,7,17,39,49,59,77,100,150,318,381,783,1731,1917,8854,11244,
%T A055520 11959,12129,18532,22717,23364,24252,24548,25323,30177,53717
%N A055520 Numbers n such that 30*R_n + 1 is prime, where R_n = 11...1 is the repunit 
               (A002275) of length n.
%C A055520 Also numbers n such that (10^(n+1)-7)/3 is prime.
%D A055520 Martin Gardner, Strong Laws of Small Primes, in The Last Recreations, 
               p. 194 (1997).
%H A055520 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               3.html">Link to a section of The World of Mathematics.</a>
%H A055520 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>
%t A055520 Do[ If[ PrimeQ[30*(10^n - 1)/9 + 1], Print[n]], {n, 0, 50410}]
%Y A055520 Equals A055557 minus 1.
%Y A055520 Indices of A033175 that are primes. Cf. A051200, A055557.
%Y A055520 Sequence in context: A065639 A039090 A138988 this_sequence A037342 A024644 
               A010354
%Y A055520 Adjacent sequences: A055517 A055518 A055519 this_sequence A055521 A055522 
               A055523
%K A055520 hard,nonn
%O A055520 1,2
%A A055520 Eric Weisstein (eric(AT)weisstein.com)