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 A127423 #33 Sep 08 2022 08:45:29
%S A127423 1,21,32,43,54,65,76,87,98,109,1110,1211,1312,1413,1514,1615,1716,
%T A127423 1817,1918,2019,2120,2221,2322,2423,2524,2625,2726,2827,2928,3029,
%U A127423 3130,3231,3332,3433,3534,3635,3736,3837,3938,4039,4140,4241,4342,4443,4544,4645
%N A127423 a(1) = 1; for n > 1, a(n) = n concatenated with n - 1.
%C A127423 a(1) could also have been defined to be 10. In that case the initial terms would be 10, 21, 32, 43, 54, 65, 76, 87, 98, 109, 1110, 1211, 1312, 1413, 1514, 1615, 1716, 1817, 1918, 2019, 2120, 2221, 2322, 2423, 2524, 2625, 2726, 2827, 2928, 3029, 3130, 3231, 3332, 3433, ... (Comment added in case someone is searching for that sequence.) - _N. J. A. Sloane_, Aug 11 2018
%C A127423 A010051(a(A054211(n))) = 1. - _Reinhard Zumkeller_, Jun 27 2015
%H A127423 Reinhard Zumkeller, <a href="/A127423/b127423.txt">Table of n, a(n) for n = 1..10000</a>
%e A127423 a(12) = 1211 because 12 and 11 are two consecutive decreasing numbers.
%p A127423 c2:=proc(x,y) local s: s:=proc(m) nops(convert(m,base,10)) end: x*10^s(y)+y: end: seq(c2(n,n-1),n=1..53); # _Emeric Deutsch_, Mar 07 2007
%t A127423 Join[{1}, nxt[n_] := Module[{idn = IntegerDigits[n + 1], idn1 = IntegerDigits[n]}, FromDigits[Join[idn, idn1]]]; Array[nxt, 70]] (* _Vincenzo Librandi_, Nov 08 2016 *)
%t A127423 nxt[{n_, a_}] := {n + 1, (n + 1) * 10^IntegerLength[n] + n}; NestList[nxt, {1, 1}, 50][[All, 2]] (* _Harvey P. Dale_, Jan 04 2019 *)
%o A127423 (Haskell)
%o A127423 a127423 n = a127423_list !! (n-1)
%o A127423 a127423_list = 1 : map read (zipWith (++) (tail iss) iss) :: [Integer]
%o A127423 where iss = map show [1..]
%o A127423 -- _Reinhard Zumkeller_, Oct 07 2014
%o A127423 (PARI) a(n) = if (n==1, 1, eval(Str(n, n-1))); \\ _Michel Marcus_, Oct 14 2016
%o A127423 (Magma) [Seqint(Intseq(n) cat Intseq(n+1)): n in [0..50]]; // _Vincenzo Librandi_, Nov 08 2016
%o A127423 (Scala) val numerStrs = (1 to 50).map(Integer.toString(_)).toList
%o A127423 val concats = (numerStrs.drop(1)) zip (numerStrs.dropRight(1))
%o A127423 concats.map(x => Integer.parseInt(x._1 + x._2)) // _Alonso del Arte_, Oct 24 2019
%Y A127423 Cf. A127421-A127424, A001704, A248378, A054211, A010051, A317947.
%K A127423 nonn,base
%O A127423 1,2
%A A127423 _Artur Jasinski_, Jan 14 2007
%E A127423 More terms from _Emeric Deutsch_, Mar 07 2007