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 A281002 #17 Feb 03 2017 21:52:23
%S A281002 3511,19,1006003,7,11,3511,491531
%N A281002 Square array read by antidiagonals downwards: A(n, 1) = second Wieferich prime to base n and A(n, k) = second Wieferich prime to base A(n, k-1) for k > 1.
%e A281002 Array starts:
%e A281002 3511, 19, 7, 491531, ...
%e A281002 1006003, 11, ...
%e A281002 3511, 19, 7, 491531, ...
%e A281002 20771, 18043, ...
%e A281002 534851, 5, 20771, 18043, ...
%o A281002 (PARI) secondwieftobase(n) = my(i=0); forprime(p=1, , if(Mod(n, p^2)^(p-1)==1, if(i==0, i++, return(p))))
%o A281002 table(rows, cols) = for(x=2, rows+1, my(i=0, w=secondwieftobase(x)); while(i < cols, print1(w, ", "); w=secondwieftobase(w); i++); print(""))
%o A281002 table(2, 3)
%Y A281002 Cf. A178871, A281001.
%K A281002 nonn,tabl,more
%O A281002 2,1
%A A281002 _Felix Fröhlich_, Jan 12 2017