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%I #59 Jan 30 2024 14:33:43
%S 11,1221,123321,12344321,1234554321,123456654321,12345677654321,
%T 1234567887654321,123456789987654321,1234567891010987654321,
%U 12345678910111110987654321,123456789101112121110987654321
%N Concatenation of the numbers from 1 to n with numbers from n down to 1.
%C For n < 700, only a(1) and a(10) are primes.
%F a(n) = A007908(n) concatenated with A000422(n).
%e a(1) = 11.
%e a(5) = 1234554321.
%t FromDigits /@ Flatten /@ IntegerDigits /@ (#~Join~Reverse@ # & /@ Table[Range@ n, {n, 12}]) (* _Michael De Vlieger_, Jul 10 2015 *)
%o (PARI) a(n)=my(r="");for(i=0,n-1,r=concat(n-i,concat(r,n-i))); eval(r); \\ _Anders Hellström_, Aug 16 2015
%o (Python)
%o from itertools import chain
%o def A259937(n): return int(''.join(str(d) for d in chain(range(1,n+1),range(n,0,-1)))) # _Chai Wah Wu_, Dec 20 2021
%Y Cf. A000422, A007908, A244861.
%K nonn,base
%O 1,1
%A _José de Jesús Camacho Medina_, Jul 09 2015
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