This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A178509 #5 Jul 17 2014 06:41:57 %S A178509 1,55,26,5,50005000,1,1,16,1936,500000000500000000,15333927,1,1,18316, %T A178509 3,7,1526,1,1,12,73,38,47,1,1,121,43502,12,11,1,1,18,3,5,10,1,1,481, %U A178509 2043419605725853,921,3835,1,1,12,10,13,25,1,1,18,3,12,62,1,1,76,398,7 %N A178509 Smallest value of k for which 6*k+1 divides the subset of centered hexagonal terms included in A177019 that admit only factors like 6*k+1. %C A178509 The terms a(0), a(1), a(4) and a(9) confirm the primality of the terms included in A160432; %C A178509 k assumes the value 1 when the value of n in a(n) is equal to 6*i or 6*i-1 where i is a positive integer. %e A178509 a(0)= 1 so 6*1+1 = 7 is the minimum factor dividing 7; a(1)= 55 so 6*55+1 = 331 the minimum factor dividing 331; a(2)= 26 so 6*26+1 = 157 the minimum factor dividing 30301; a(3)= 5 so 6*5+1 = 31 the minimum factor dividing 3003001; a(10)=15333927 so 6*15333927+1 = 92003563 the minimum factor dividing 3*10^20+3*10^10+1. %Y A178509 Cf. A177019, A160432. %K A178509 nonn %O A178509 0,2 %A A178509 _Giacomo Fecondo_, May 29 2010, May 30 2010