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 A133780 #9 Mar 11 2014 01:32:22 %S A133780 1,2,1,2,1,2,3,1,2,1,2,1,2,3,4,1,2,1,2,1,2,3,1,2,4,5,1,2,1,2,3,4,1,2, %T A133780 1,2,1,2,3,5,6,1,2,1,2,1,2,3,4,1,2,1,2,4,5,1,2,3,6,7,1,2,1,2,1,2,3,4, %U A133780 1,2,1,2,1,2,3,1,2,7,8,1,2,1,2,3,4,5,6,1,2,1,2,1,2,3,1,2,1,2,1,2,3,4,8,9,1 %N A133780 Irregular array: n-th row lists the "non-isolated divisors" of (2n). A positive divisor, k, of n is non-isolated if (k-1) or (k+1) also divides n. %C A133780 No odd integer has any non-isolated divisors. The number of terms in the n-th row of the array is A132747(2n). %e A133780 The positive divisors of 20 are 1,2,4,5,10,20. Of these, 1 and 2 are adjacent and 4 and 5 are adjacent. So the non-isolated divisors of 20 are 1,2,4,5. %Y A133780 Cf. A133779, A132747, A132748. %K A133780 nonn,tabf %O A133780 1,2 %A A133780 _Leroy Quet_, Sep 23 2007 %E A133780 Extended by _Ray Chandler_, Jun 24 2008