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 A120403 #4 Mar 30 2012 18:36:04 %S A120403 3,9,15,45,225,639,1275,4005,675405,2203959,3075159,6195234165, %T A120403 77989711185,4566262987329 %N A120403 a(1)=3; a(n)=first odd number greater than a(n-1) such that 2*a(n)-1 is prime and a(i)+a(n)-1 is prime for all 1<=i<=n-1. %C A120403 All elements are 3 mod 6. In base 12 the sequence is 3, 9, 13, 39, 169, 453, 8X3, 2399, 286X39, 8X3533, 1043733, where X is 10 and E is eleven. %F A120403 a(1)=3; a(n) = s where s is the first odd number s>a(n-1) such that 2*s-1 is prime and s+a(i)-1 is prime, 1<=i<=n-1. %F A120403 a(n) = A119752(n) + 1. - Chandler %e A120403 a(2)=9 since 9 is the first odd number > a(1)=3 such that 2*9-1=17 is prime and 9+3-1=13 is prime. %p A120403 OP:=[3]: for w to 1 do for k from 0 to 12^8 do n:=6*k+3; p:=2*n-1; Q:=map(z-> z+n-1, OP); if isprime(p) and andmap(isprime,Q) then OP:=[op(OP), n]; print(n); fi od od; %Y A120403 Cf. A119752, A119754 (resulting primes), A119751, A119753, A103828. %K A120403 nonn,more %O A120403 1,1 %A A120403 _Walter Kehowski_, Jul 02 2006 %E A120403 a(12)-a(14) from _Ray Chandler_, Apr 04 2010