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 A069708 #18 Feb 01 2021 22:09:52
%S A069708 1,3,6,10,55,66,120,153,171,190,300,351,595,630,666,820,1081,1431,
%T A069708 1711,1891,3003,3403,5050,5460,5565,5995,6216,6786,8128,8778,10011,
%U A069708 10731,11781,12561,13041,13861,15051,15931,16471,17020,17391,17578,18721
%N A069708 Triangular numbers with property that swapping first and last digits also gives a triangular number.
%C A069708 934 of the first 1000 terms begin and end with the same digit. 40 of the first 1000 terms end in zero. Thus, only 26 of the first 1000 terms begin and end with different nonzero digits, with 153 being the smallest and 8026021 being the largest of those terms. - _Harvey P. Dale_, Jan 09 2021
%H A069708 Harvey P. Dale, <a href="/A069708/b069708.txt">Table of n, a(n) for n = 1..1000</a>
%e A069708 820 and 028 = 28 both are triangular numbers hence both are members.
%t A069708 Do[t = IntegerDigits[n(n + 1)/2]; u = t; u[[1]] = t[[ -1]]; u[[ -1]] = t[[1]]; t = FromDigits[u]; u = Floor[ Sqrt[2t]]; If[ u(u + 1)/2 == t, Print[n(n + 1)/2]], {n, 1, 300}]
%t A069708 sfl[n_]:=Module[{idn=IntegerDigits[n]},FromDigits[Flatten[Join[{Last[ idn],Rest[ Most[ idn]],First[ idn]}]]]]; Join[ {1,3,6},Select[ Accumulate[ Range[200]],OddQ[Sqrt[8 sfl[#]+1]]&]//Quiet] (* _Harvey P. Dale_, Jan 09 2021 *)
%Y A069708 Cf. A069706, A069707.
%K A069708 nonn,base
%O A069708 1,2
%A A069708 _Amarnath Murthy_, Apr 08 2002
%E A069708 Edited, corrected and extended by _Robert G. Wilson v_
%E A069708 Edited by _N. J. A. Sloane_, Jan 20 2009