%I A090464
%S A090464 1,0,0,1,0,1,0,2,1,1,0,1,0,1,1,1,0,2,0,6,1,1,0,2,1,2,1,1,0,1,0,5,1,1,1,
%T A090464 1,0,2,1,12,0,1,0,3,1,1,0,1,1,2,8,7,0,1,1,1,1,1,0,1,0,2,1,1,2,5,0,3,2,
               1,
%U A090464 0,1,0,2,1,3,1,1,0,3,4,1,0,1,1,2,1,1,0,1,1,2,1,1
%V A090464 1,0,0,1,0,1,0,2,1,1,0,1,0,-1,1,1,0,2,0,6,-1,1,0,2,1,2,1,-1,0,1,0,5,1,
               1,-1,1,0,2,1,12,
%W A090464 0,-1,0,3,1,1,0,1,-1,2,8,7,0,1,1,-1,1,1,0,1,0,2,-1,1,2,5,0,3,2,-1,0,1,
               0,2,1,3,-1,1,0,3,
%X A090464 4,1,0,-1,1,2,1,1,0,1,-1,2,1,1
%N A090464 Smallest number m such that n followed by m sevens yields a prime or 
               -1 if no solution exists or has been found for n.
%C A090464 a(n) = 0 if n is already prime. a(n) = -1 for n = any multiple of 7 other 
               than 7 itself. Each multiple of 7 has been tested out to 2000 7's 
               with no result found. The first eight values of n which are not multiples 
               of 7 for which no answer has yet been found are 95, 480, 851, 891, 
               957, 1184, 1261, 1881. 95 has been tested out to 2100 7's, 1881 has 
               been tested out to 1750 7's, the others have been tested out to 2000 
               7's. Pending solutions for these values of n, the first 10 record 
               holders are currently 1, 8, 20, 40, 120, 128, 225, 260, 296, 711 
               with the values 1, 2, 6, 12, 16, 18, 56, 182, 434, 1648 respectively.
%e A090464 a(20)=6 because six 7's must be appended to 20 before a prime is formed 
               (20777777). a(14) = -1 because no matter how many 7's are appended 
               to 14, the resulting number is always divisible by 7 and can therefore 
               not be prime.
%Y A090464 Cf. A083747 (The Wilde Primes, i.e. same operation using ones), A090465 
               (using nines), A090584 (using threes).
%Y A090464 Sequence in context: A117208 A133300 A144451 this_sequence A044934 A124761 
               A156709
%Y A090464 Adjacent sequences: A090461 A090462 A090463 this_sequence A090465 A090466 
               A090467
%K A090464 base,sign
%O A090464 1,8
%A A090464 Chuck Seggelin (barkeep(AT)plastereddragon.com), Dec 02 2003