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 A138797 #3 Jun 29 2025 22:16:43
%S A138797 3,6,10,6,21,10,36,10,55,21,15,28,15,21,136,45,21,55,21,36,28,78,45,
%T A138797 28,36,28,406,120,36,136,528,36,55,36,91,190,66,45,55,231,45,253,45,
%U A138797 55,91,300,153,55,78,66,55,378,55,91,66,78,136,465,66,496,153,66,2080,66,171
%N A138797 Least possible T(k) with T(k)-T(j)=n, where T(i)>0 are the triangular numbers A000217.
%C A138797 For k see A138796, for j see A138798 and for T(j) see A138799.
%C A138797 The number of ways n can be written as difference of two triangular numbers is sequence A136107
%e A138797 a(4)=10 because T(A138796(4))=10.
%t A138797 T=#(#+1)/2&;T[Min[k/.{ToRules[Reduce[{T[k]-T[j]\[Equal]#,0<j<k},{j,k},Integers]]}]]&/@Range[2,100]
%Y A138797 Cf. A000217, A109814, A118235, A136107, A138796, A138798, A138799.
%K A138797 nonn
%O A138797 2,1
%A A138797 Peter Pein (petsie(AT)dordos.net), Mar 30 2008