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%I #25 Sep 08 2022 08:45:33
%S 1,21,321,4321,54321,654321,7654321,87654321,987654321,1987654321,
%T 1101987654321,211101987654321,31211101987654321,4131211101987654321,
%U 514131211101987654321,61514131211101987654321,7161514131211101987654321
%N a(n) = concatenation of reversed digits of natural numbers from n down to 1.
%C Note that leading zeros are not omitted when writing down digits in reversed order. So 10 reversed becomes 01. - _N. J. A. Sloane_, Jan 23 2017
%H Vincenzo Librandi, <a href="/A138793/b138793.txt">Table of n, a(n) for n = 1..100</a>
%p read(transforms): A138793 := proc(n) return digrev(parse(cat($(1..n)))): end: seq(A138793(n),n=1..17); # _Nathaniel Johnston_, Jun 26 2011
%t b = {}; a = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[k]]], {k, 1, Length[w]}]; p = FromDigits[Reverse[a]]; AppendTo[b, p], {n, 1, 61}]; b (* _Artur Jasinski_, Mar 30 2008 *)
%t lst = {}; Table[FromDigits[Reverse[lst = Join[lst, IntegerDigits[n]]]], {n, 1, 15}] (* _Robert Price_, Mar 22 2015 *)
%o (Magma) [Seqint(&cat[Reverse(Intseq(k)): k in [1..n]]): n in [1..16]]; // _Bruno Berselli_, May 27 2011
%o (PARI) a(n) = my(s = ""); forstep (k=n,1,-1, sk = digits(k); forstep (j=#sk, 1, -1, s = concat(s, sk[j]))); eval(s); \\ _Michel Marcus_, Jan 28 2017
%Y Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790.
%K nonn,easy,base
%O 1,2
%A _Artur Jasinski_, Mar 30 2008, Apr 04 2008
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