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 A154721 #11 Dec 04 2017 08:36:31
%S A154721 0,1,0,3,1,0,0,0,5,1,0,3,0,5,0,7,0,0,3,0,0,0,7,0,0,1,0,0,0,5,0,7,0,0,
%T A154721 0,11,1,0,3,0,0,0,0,0,0,0,11,0,13,0,0,3,0,5,0,0,0,0,0,11,0,13,0,0,1,0,
%U A154721 0,0,5,0,7,0,0,0,11,0,13,0,0,0,17
%N A154721 Triangle read by rows in which row n lists 2n-1 terms: The pairs of noncomposite numbers equidistant to n, with 0's inserted, as shown below in the example.
%H A154721 Nathaniel Johnston, <a href="/A154721/b154721.txt">Table of n, a(n) for n = 1..10000</a>
%e A154721 Triangle begins:
%e A154721 0
%e A154721 1 0 3
%e A154721 1 0 0 0 5
%e A154721 1 0 3 0 5 0 7
%e A154721 0 0 3 0 0 0 7 0 0
%e A154721 1 0 0 0 5 0 7 0 0 0 11
%e A154721 1 0 3 0 0 0 0 0 0 0 11 0 13
%e A154721 0 0 3 0 5 0 0 0 0 0 11 0 13 0 0
%e A154721 1 0 0 0 5 0 7 0 0 0 11 0 13 0 0 0 17
%e A154721 1 0 3 0 0 0 7 0 0 0 0 0 13 0 0 0 17 0 19
%p A154721 isnotcomp:=proc(n)return (n=1 or isprime(n)) end:
%p A154721 for n from 1 to 10 do for k from 1 to 2*n-1 do if(not k=n and (isnotcomp(k) and isnotcomp(2*n-k)))then print(k):else print(0):fi:od:od: # _Nathaniel Johnston_, Apr 18 2011
%t A154721 T[n_, k_] := If[k != n && !CompositeQ[k] && !CompositeQ[2n - k], k, 0];
%t A154721 Table[T[n, k], {n, 1, 10}, {k, 1, 2n - 1}] // Flatten (* _Jean-François Alcover_, Dec 04 2017 *)
%Y A154721 Cf. A000040, A008578, A154720, A154722, A154723, A154724, A154725, A154726, A154727.
%K A154721 easy,nonn,tabf
%O A154721 1,4
%A A154721 _Omar E. Pol_, Jan 14 2009