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%I #19 Mar 31 2015 00:45:16
%S 1,21,321,4321,54321,654321,7654321,87654321,987654321,1987654321,
%T 1987654321,101987654321,1101987654321,11101987654321,211101987654321,
%U 1211101987654321,31211101987654321,131211101987654321
%N Concatenation of digits of natural numbers from n down to 1.
%H Michael De Vlieger, <a href="/A104759/b104759.txt">Table of n, a(n) for n = 1..1000 (last term has 1000 decimal digits)</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ConsecutiveNumberSequences.html">Consecutive numbers sequences</a>.
%F a(n) = A138793(n) mod 10^(n-1). - _R. J. Mathar_, Sep 17 2011
%e a(11) = a(10) because no number may begin with 0.
%e a(9)= [123456789]101112131415...=987654321
%e a(10)=[1234567891]01112131415...=1987654321
%e a(11)=[12345678910]1112131415...=01987654321=1987654321
%e a(12)=[123456789101]112131415...=101987654321
%e a(13)=[1234567891011]12131415...=1101987654321
%e a(14)=[12345678910111]2131415...=11101987654321
%e a(15)=[123456789101112]131415...=211101987654321
%t f[n_] := Block[{t = Reverse@ Flatten@ IntegerDigits@ Range@ n, k}, Reap@ For[k = 1, k <= Length@ t, k++, Sow[FromDigits@ Take[t, -k]]] // Flatten // Rest]; f@ 14 (* _Michael De Vlieger_, Mar 23 2015 *)
%t lst = {}; Do[lst = Join[lst, IntegerDigits[n]], {n, 1, 100}]; Table[FromDigits[Reverse[lst[[Range[1, n]]]]], {n, 1, Length[lst]}] (* _Robert Price_, Mar 24 2015 *)
%Y Cf. A014925, A000422, A057138, A060554...
%K easy,nonn,base
%O 1,2
%A _Alexandre Wajnberg_ & Juliette Bruyndonckx, Apr 23 2005
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