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 A281001 #19 Feb 03 2017 21:51:54
%S A281001 1093,2,11,1093,71,1093,2,3,2,2,1093,11,1093,1093,66161,2,71,2,2,2,5,
%T A281001 1093,3,1093,1093,1093,2,3,2,11,2,2,2,1093,11,2,1093,71,1093,1093,
%U A281001 1093,2,71,1093,3,2,3,2,2,2,1093,3,2,11,71,1093,11,1093,1093,1093,2
%N A281001 Square array read by antidiagonals downwards: A(n, 1) = smallest Wieferich prime to base n and A(n, k) = smallest Wieferich prime to base A(n, k-1) for k > 1.
%C A281001 Row n becomes periodic, repeating the terms 2, 1093 if n is in A252801 when n is prime or if A039951(n) is in A252801 when n is composite.
%C A281001 Row n becomes periodic, repeating the terms 3, 11, 71 if n is in A252802 when n is prime or if A039951(n) is in A252802 when n is composite.
%C A281001 Row n becomes periodic, repeating the terms 83, 4871 if n is in A252812 when n is prime or if A039951(n) is in A252812 when n is composite.
%e A281001 Array starts
%e A281001 1093, 2, 1093, 2, 1093, 2, ...
%e A281001 11, 71, 3, 11, 71, 3, ...
%e A281001 1093, 2, 1093, 2, 1093, 2, ...
%e A281001 2, 1093, 2, 1093, 2, 1093, ...
%e A281001 66161, 2, 1093, 2, 1093, 2, ...
%e A281001 5, 2, 1093, 2, 1093, 2, ...
%e A281001 ....
%o A281001 (PARI) smallestwieftobase(n) = forprime(p=1, , if(Mod(n, p^2)^(p-1)==1, return(p)))
%o A281001 table(rows, cols) = for(x=2, rows+1, my(i=0, w=smallestwieftobase(x)); while(i < cols, print1(w, ", "); w=smallestwieftobase(w); i++); print(""))
%o A281001 table(7, 5) \\ print initial 5 terms of upper 7 rows of array
%Y A281001 Cf. A039951, A252801, A252802, A252812, A281002.
%K A281001 nonn,tabl
%O A281001 2,1
%A A281001 _Felix Fröhlich_, Jan 12 2017