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 A061881 #8 May 19 2019 11:34:55
%S A061881 309,408,419,507,518,529,606,617,628,639,705,716,727,738,749,804,815,
%T A061881 826,837,848,859,903,914,925,936,947,958,969,3090,4080,4091,4190,5070,
%U A061881 5081,5092,5180,5191,5290,6060,6071,6082,6093,6170,6181,6192,6280,6291
%N A061881 First (leftmost) digit - second digit + third digit - fourth digit .... = 12.
%t A061881 okQ[n_]:=Plus@@(Times@@#&/@Partition[Most[Riffle[IntegerDigits[10n],{1,-1}]],2])==12; Select[Range[7000],okQ] (* _Harvey P. Dale_, Jan 17 2011 *)
%t A061881 okQ[n_] := Module[{d = IntegerDigits[n]}, Plus @@ Take[d, {1, Length[d], 2}] - Plus @@ Take[d, {2, Length[d], 2}] == 12]; Select[Range[7000], okQ]
%Y A061881 Cf. A008593, A060978-A060980, A060982, A061470-A061479, A061870-A061882.
%K A061881 base,nonn,easy
%O A061881 1,1
%A A061881 Larry Reeves (larryr(AT)acm.org), May 15 2001