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 A129120 #11 Jan 27 2021 05:35:13
%S A129120 10,12,18,20,21,24,27,30,36,40,42,45,48,50,54,60,63,70,72,80,81,84,90,
%T A129120 171,252,414,828,1010,1212,1818,2020,2424,3030,3636,4040,4848,5050,
%U A129120 5454,6060,7070,7272,8080,9090,10101,13131,20202,23232,26262,30303,39393
%N A129120 Undulating Harshad numbers: numbers divisible by the sum of their own digits with decimal expansions in an abab...ab pattern.
%C A129120 This definition of "undulating" is more restrictive than the one used in A033619 and excludes single-digit numbers and numbers of the pattern aaaaaa. - _R. J. Mathar_, Jun 15 2007
%D A129120 Jason Earls, Red Zen, Lulu Press, NY, 2006, pp. 56-57. ISBN: 978-1-4303-2017-3.
%H A129120 Amiram Eldar, <a href="/A129120/b129120.txt">Table of n, a(n) for n = 1..2146</a>
%t A129120 undHarsQ[n_] := Module[{d = IntegerDigits[n]}, Divisible[n, Plus @@ d] && Length @ Union[d] > 1 && Length @ Union[d[[1 ;; -1 ;; 2]]] == 1 && Length @ Union[d[[2 ;; -1 ;; 2]]] == 1 ]; Select[Range[40000], undHarsQ] (* _Amiram Eldar_, Jan 27 2021 *)
%Y A129120 Cf. A005349, A033619.
%K A129120 base,nonn
%O A129120 1,1
%A A129120 _Jason Earls_, May 25 2007
%E A129120 More terms from _R. J. Mathar_, Jun 15 2007