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 A078795 #9 Jan 13 2021 21:27:47
%S A078795 1,13,136,13610,1361015,136101521,13610152128,1361015212836,
%T A078795 136101521283645,13610152128364555,1361015212836455566,
%U A078795 136101521283645556678,13610152128364555667891,13610152128364555667891105,13610152128364555667891105120
%N A078795 Concatenate first n triangular numbers.
%C A078795 There are only 2 primes i.e. 13 and 136101521 known in the sequence for first 1000 terms of the sequence.
%D A078795 Shyam Sunder Gupta, Smarandache Sequence of Triangular Numbers, Smarandache Notions Journal, (to appear in Vol. 14, 2003).
%e A078795 a(3)=136 because 1, 3, 6 are first 3 triangular numbers.
%p A078795 a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(a(n-1), n*(n+1)/2))) end:
%p A078795 seq(a(n), n=1..20); # _Alois P. Heinz_, Jan 13 2021
%t A078795 Table[FromDigits[Flatten[IntegerDigits[Table[n*(n+1)/2, {n, 1, m}]]]], {m, 1, 20}]
%Y A078795 Cf. A000217.
%K A078795 base,easy,nonn
%O A078795 1,2
%A A078795 _Shyam Sunder Gupta_, Jan 10 2003