cp's OEIS Frontend

A281254 Concatenate the decimal numbers n n-2 n-4 ...5 3 1 2 4 ... n-5 n-3 n-1 if n is odd, or n n-2 n-4 ... 6 4 2 1 3 5 ... n-5 n-3 n-1 if n is even.

%I A281254 #28 May 22 2025 10:21:45
%S A281254 1,21,312,4213,53124,642135,7531246,86421357,975312468,10864213579,
%T A281254 1197531246810,121086421357911,13119753124681012,1412108642135791113,
%U A281254 151311975312468101214,16141210864213579111315,1715131197531246810121416
%N A281254 Concatenate the decimal numbers n n-2 n-4 ...5 3 1 2 4 ... n-5 n-3 n-1 if n is odd, or n n-2 n-4 ... 6 4 2 1 3 5 ... n-5 n-3 n-1 if n is even.
%C A281254 The old name was "Alternately concatenate the decimal digits from back to front 1...n such that n is always to the left.".
%C A281254 a(40714), with 192464 decimal digits, is the sequence's first (probable) prime. - _Hans Havermann_, Dec 17 2019
%C A281254 The terms at even indices are the same as the even-indexed terms of A053063. - _Hans Havermann_, Jan 16 2020
%H A281254 Indranil Ghosh, <a href="/A281254/b281254.txt">Table of n, a(n) for n = 1..340</a>
%e A281254 a(13) = 13119753124681012.
%p A281254 b:= proc(n) b(n):= `if`(n=1, 1, parse(cat(a(n-1), n))) end:
%p A281254 a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(n, b(n-1)))) end:
%p A281254 seq(a(n), n=1..20);  # _Alois P. Heinz_, Jan 18 2017
%t A281254 f[n_] := Fold[ If[ Mod[n +#2, 2] == 0, #2, #1]*10^IntegerLength@ If[ Mod[n +#2, 2] == 0, #1, #2] +If[ Mod[n +#2, 2] == 1, #2, #1] &, 0, Range@ n]; Array[f, 17]
%o A281254 (Python)
%o A281254 def a(n):
%o A281254     if n==1:
%o A281254         return ["1"]
%o A281254     return [str(n)]+a(n-1)[::-1]
%o A281254 def A281254(n):
%o A281254     return "".join(a(n)) # _Indranil Ghosh_, Jan 23 2017
%Y A281254 See A053063 for another version.
%Y A281254 Cf. A000422, A007908, A138793, A138957, A281253.
%K A281254 nonn,base
%O A281254 1,2
%A A281254 _Robert G. Wilson v_, Jan 18 2017
%E A281254 Edited by _N. J. A. Sloane_, Dec 07 2019