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 A173539 #2 Mar 30 2012 17:34:04 %S A173539 0,0,2,0,0,3,0,2,3,4,0,0,0,4,5,0,2,3,4,5,6,0,0,3,0,5,6,7,0,2,0,4,5,6, %T A173539 7,8,0,0,3,4,0,6,7,8,9,0,2,3,4,5,6,7,8,9,10,0,0,0,0,5,0,7,8,9,10,11,0, %U A173539 2,3,4,5,6,7,8,9,10,11,12,0,0,3,4,5,6,0,8,9,10,11,12,13,0,2,0,4,0,6,7,8,9 %N A173539 Square array read by antidiagonals: T(n,k)=0 if k is a divisor of n, otherwise T(n,k)=k. %C A173539 Observation: Column k is the sequence defined as: Period k: repeat (k-1 numbers k together with zero). %C A173539 Note that the positive terms in row n are the non-divisors of n (See also A173540 and A173541). %e A173539 Array begins: %e A173539 0,2,3,4,5,6,7,8,9,10,11,12; %e A173539 0,0,3,4,5,6,7,8,9,10,11; %e A173539 0,2,0,4,5,6,7,8,9,10; %e A173539 0,0,3,0,5,6,7,8,9; %e A173539 0,2,3,4,0,6,7,8; %e A173539 0,0,0,4,5,0,7; %e A173539 0,2,3,4,5,6; %e A173539 0,0,3,0,5; %e A173539 0,2,0,4; %e A173539 0,0,3; %e A173539 0,2; %e A173539 0; %Y A173539 Cf. A007978, A173540, A173541. %K A173539 easy,nonn,tabl %O A173539 1,3 %A A173539 _Omar E. Pol_, May 25 2010