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 A143473 #12 May 25 2020 17:33:56
%S A143473 10,9,8,7,6,5,4,3,2,1,90,91,92,93,94,95,96,97,98,99,80,81,82,83,84,85,
%T A143473 86,87,88,89,70,71,72,73,74,75,76,77,78,79,60,61,62,63,64,65,66,67,68,
%U A143473 69,50,51,52,53,54,55,56,57,58,59,40,41,42,43,44,45,46,47,48,49,30,31
%N A143473 Replace the leading digit d of n with 10-d (in decimal representation).
%C A143473 a(n) = n iff A000030(n) = 5, cf. A217398.
%H A143473 R. Zumkeller, <a href="/A143473/b143473.txt">Table of n, a(n) for n = 0..9999</a>
%F A143473 A000030(a(n)) = 10 - A000030(n).
%t A143473 ld10[n_]:=Module[{idn=IntegerDigits[n]},FromDigits[Join[{10-idn[[1]]},Rest[ idn]]]]; Array[ld10,100,0] (* _Harvey P. Dale_, May 25 2020 *)
%o A143473 (Haskell)
%o A143473 a143473 n = foldl (\v d -> 10 * v + d) 0 $ (10 - z) : zs where
%o A143473 (z:zs) = map (read . return) $ show n
%K A143473 nonn,base,look
%O A143473 0,1
%A A143473 _Reinhard Zumkeller_, Aug 18 2008