A259937 Concatenation of the numbers from 1 to n with numbers from n down to 1.

11, 1221, 123321, 12344321, 1234554321, 123456654321, 12345677654321, 1234567887654321, 123456789987654321, 1234567891010987654321, 12345678910111110987654321, 123456789101112121110987654321

Offset

1,1

Comments

For n < 700, only a(1) and a(10) are primes.

Links

Table of n, a(n) for n=1..12.

Formula

a(n) = A007908(n) concatenated with A000422(n).

Example

a(1) = 11.
a(5) = 1234554321.

Mathematica

FromDigits /@ Flatten /@ IntegerDigits /@ (#~Join~Reverse@ # & /@ Table[Range@ n, {n, 12}]) (* Michael De Vlieger, Jul 10 2015 *)

Prog

(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
(Python)
from itertools import chain
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

Crossrefs

Cf. A000422, A007908, A244861.

Sequence in context: A019524 A274766 A261138 * A078269 A015009 A068326

Adjacent sequences: A259934 A259935 A259936 * A259938 A259939 A259940

Keyword

nonn,base

Author

José de Jesús Camacho Medina, Jul 09 2015

Status

approved