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 A020331 #23 Apr 06 2021 10:00:29
%S A020331 4,8,30,40,50,60,70,80,252,280,308,336,364,392,420,448,476,504,532,
%T A020331 560,588,616,644,672,700,728,2214,2296,2378,2460,2542,2624,2706,2788,
%U A020331 2870,2952,3034,3116,3198,3280,3362,3444,3526,3608,3690,3772,3854,3936,4018
%N A020331 Numbers whose base-3 representation is the juxtaposition of two identical strings.
%H A020331 Amiram Eldar, <a href="/A020331/b020331.txt">Table of n, a(n) for n = 1..10000</a>
%F A020331 a(n) = n*3^floor(log_3(n)+1) + n. - _Ilya Gutkovskiy_, Jan 26 2018
%e A020331 50_10 = 1212_3. - _Jon E. Schoenfield_, Feb 11 2021
%t A020331 b3iQ[n_]:=Module[{idn3=IntegerDigits[n,3],len},len=Length[idn3];EvenQ[ len] && Take[idn3,len/2]==Take[idn3,-len/2 ]]; Select[Range[5000],b3iQ] (* _Harvey P. Dale_, Feb 08 2015 *)
%t A020331 a[n_] := n + n*3^Floor[Log[3, n] + 1]; Array[a, 50] (* _Amiram Eldar_, Apr 06 2021 *)
%Y A020331 Cf. A020330, A020332, A020333, A020334, A020335, A020336, A020337, A020338.
%K A020331 nonn,base
%O A020331 1,1
%A A020331 _David W. Wilson_, Melia Aldridge (ma38(AT)spruce.evansville.edu)