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 A152989 #12 May 06 2014 14:09:19 %S A152989 0,0,3,5,6,8,23,47,28,14,71,83,18,80,225,127,76,84,163,351,146,32,385, %T A152989 551,104,202,567,307,278,296,487,941,296,262,1219,805,54,372,1549,933, %U A152989 476,498,631,1795,826,68,1737,2221,534,946,1683,883,722,1380,2469,2861,740,86,2535 %N A152989 Sum of proper divisors minus the number of proper divisors of triangular number A000217(n). %H A152989 B. D. Swan, <a href="/A152989/b152989.txt">Table of n, a(n) for n=1,...,10000</a> %F A152989 a(n) = A001065(A000217(n)) - A032741(A000217(n)) = A152770(A000217(n)). %t A152989 DivisorSigma[1,#]-DivisorSigma[0,#]-#+1&/@Accumulate[Range[60]] (* _Harvey P. Dale_, May 06 2014 *) %o A152989 (PARI) a(n) = sigma(n*(n+1)/2) - n*(n+1)/2 - (numdiv(n*(n+1)/2) - 1); \\ _Michel Marcus_, Jan 28 2014 %Y A152989 Cf. A000005, A000203, A000217, A001065, A032741, A152770. %K A152989 nonn %O A152989 1,3 %A A152989 _Omar E. Pol_, Dec 20 2008