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 A241241 #14 Aug 12 2016 17:48:44
%S A241241 0,1,2,3,4,6,9,10,16,21,36,45,55,81,100,136,231,256,441,666,1035,1296,
%T A241241 1540,2025,3025,3321,5050,6561,9316,10000,18496,26796,32896,53361,
%U A241241 65536,97461,194481,222111,443556,536130,840456,1071225,1186570,1679616,2051325
%N A241241 If x is in the sequence then so are x^2 and x(x+1)/2.
%H A241241 Reinhard Zumkeller, <a href="/A241241/b241241.txt">Table of n, a(n) for n = 1..1000</a>
%t A241241 Nest[Flatten[{#,#^2,(#(#+1))/2}]&,{0,1,2},5]//Union (* _Harvey P. Dale_, Aug 12 2016 *)
%o A241241 (Haskell)
%o A241241 import Data.Set (singleton, deleteFindMin, insert)
%o A241241 a241241 n = a241241_list !! (n-1)
%o A241241 a241241_list = 0 : 1 : f (singleton 2) where
%o A241241 f s = m : f (insert (a000290 m) $ insert (a000217 m) s')
%o A241241 where (m, s') = deleteFindMin s
%Y A241241 Cf. A000290, A000217.
%Y A241241 Subsequence of A005214; some subsequences: A001146, A007501, A011764, A176594, A173501, A050909.
%K A241241 nonn
%O A241241 1,3
%A A241241 _Reinhard Zumkeller_, Apr 17 2014
%E A241241 Initial 0 and 1 prepended by _Jon Perry_, Apr 17 2014