%I A101392
%S A101392 1,5,25,455,761,9205,13561,15955
%N A101392 Numbers n such that 2*10^n+9 is prime.
%C A101392 761 and 9205 only probably prime. No others less than 10000.
%H A101392 Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/20009.htm">
               Factorizations of 200...009</a>.
%H A101392 Sabin Tabiraca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/
               Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%H A101392 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>.
%e A101392 n = 1, 5 are members since 29 and 200009 are primes.
%t A101392 Do[ If[ PrimeQ[2*10^n + 9], Print[n]], {n, 0, 10000}]
%Y A101392 Cf. A081677, A088275.
%Y A101392 Sequence in context: A145773 A072324 A082026 this_sequence A078260 A007185 
               A030995
%Y A101392 Adjacent sequences: A101389 A101390 A101391 this_sequence A101393 A101394 
               A101395
%K A101392 hard,nonn
%O A101392 1,2
%A A101392 Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005
%E A101392 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008