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A007942 Decimal concatenation of sequence (n, n-1, ..., 2, 1, 2, ..., n-1, n).

%I A007942 #46 Jan 01 2024 02:18:59
%S A007942 1,212,32123,4321234,543212345,65432123456,7654321234567,
%T A007942 876543212345678,98765432123456789,109876543212345678910,
%U A007942 1110987654321234567891011,12111098765432123456789101112,131211109876543212345678910111213,1413121110987654321234567891011121314
%N A007942 Decimal concatenation of sequence (n, n-1, ..., 2, 1, 2, ..., n-1, n).
%C A007942 For n <= 1530, only a(13) = 131211109876543212345678910111213 is prime. - _XU Pingya_, May 21 2017
%H A007942 Michael S. Branicky, <a href="/A007942/b007942.txt">Table of n, a(n) for n = 1..202</a>
%H A007942 F. Smarandache, <a href="http://www.gallup.unm.edu/~smarandache/OPNS.pdf">Only Problems, Not Solutions!</a>
%p A007942 a:= n-> parse(cat(n-i$i=0..n-1, $2..n)):
%p A007942 seq(a(n), n=1..23);  # _Alois P. Heinz_, Dec 19 2021
%t A007942 Table[d = Flatten[IntegerDigits /@ Range@ n]; FromDigits@ Flatten[{Reverse@ d, Rest@ d}, 1], {n, 11}] (* _Michael De Vlieger_, Aug 20 2015 *)
%o A007942 (PARI) a(n) = s = ""; forstep (k=n,1,-1, s = concat(s, k)); for (k=2, n, s = concat(s, k)); eval(s); \\ _Michel Marcus_, Aug 20 2015
%o A007942 (Python)
%o A007942 from itertools import count, islice
%o A007942 def agen(): # generator of terms
%o A007942     s = "1"
%o A007942     for n in count(2):
%o A007942         yield int(s)
%o A007942         s = str(n) + s + str(n)
%o A007942 print(list(islice(agen(), 14))) # _Michael S. Branicky_, Dec 09 2022
%o A007942 (Python)
%o A007942 def A007942(n): return int(''.join(map(str,range(n,1,-1)))+''.join(map(str,range(1,n+1)))) # _Chai Wah Wu_, Mar 21 2023
%K A007942 nonn,base,easy
%O A007942 1,2
%A A007942 R. Muller