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 A138799 #4 Dec 23 2018 03:45:53
%S A138799 1,3,6,1,15,3,28,1,45,10,3,15,1,6,120,28,3,36,1,15,6,55,21,3,10,1,378,
%T A138799 91,6,105,496,3,21,1,55,153,28,6,15,190,3,210,1,10,45,253,105,6,28,15,
%U A138799 3,325,1,36,10,21,78,406,6,435,91,3,2016,1,105,528,10,36,21,595,6,630
%N A138799 Values of T(j) corresponding to least possible T(k) with T(k)-T(j)=n, where T(i)>0 are the triangular numbers A000217.
%C A138799 For k see A138796, for T(k) see A138797 and for j see A138798.
%C A138799 The number of ways n can be written as difference of two triangular numbers is sequence A136107
%e A138799 a(30)=6 because 30 = T(30)-T(29)=T(11)-T(8)=T(9)-T(5)=T(8)-T(3) and T(3)=6 is the least minuend.
%t A138799 T=#(#+1)/2&;T[Sort[{k,j}/.{ToRules[Reduce[{T[k]-T[j]\[Equal]#,0<j<k},{j,k},Integers]]}][[1,2]]]&/@Range[2,100]
%Y A138799 Cf. A000217, A109814, A118235, A136107, A138796, A138797, A138798.
%K A138799 nonn
%O A138799 2,2
%A A138799 Peter Pein (petsie(AT)dordos.net), Mar 30 2008