A007907 Concatenation of sequence (1, 2, ..., floor((n-1)/2), floor(n/2), floor(n/2)-1, ..., 1) for n >= 1.

1, 11, 121, 1221, 12321, 123321, 1234321, 12344321, 123454321, 1234554321, 12345654321, 123456654321, 1234567654321, 12345677654321, 123456787654321, 1234567887654321, 12345678987654321

Offset

1,2

Comments

For n < 4900, a(2) = A259937(1) = 11, a(19) = A173426(10) = 12345678910987654321, a(20) = A259937(10) = 1234567891010987654321 and a(4891) = A173426(2446) = 1234567..244524462445..7654321 are primes (see A173426 and A259937). - XU Pingya, May 19 2017

References

M. Le, The Primes in the Smarandache Symmetric Sequences, Smarandache Notions Journal, Vol. 10, No. 1-2-3, 1999, 174-175.

Links

Michael De Vlieger, Table of n, a(n) for n = 1..405 [The list of terms has an interesting visual appearance]

F. Smarandache, Only Problems, Not Solutions!

Eric Weisstein's World of Mathematics, Smarandache Sequences

Mathematica

Table[FromDigits@ Flatten@ Map[IntegerDigits, Apply[Join, {#, If[OddQ@ n, Rest@ #, #] &@ Reverse@ #}]] &@ Range@ Ceiling[n/2]], {n, 17}] (* Michael De Vlieger, May 20 2017 *)

Crossrefs

Cf. A173426, A259937.

Sequence in context: A125315 A223676 A132583 * A171285 A216132 A088113

Adjacent sequences: A007904 A007905 A007906 * A007908 A007909 A007910

Keyword

nonn,base

Author

R. Muller

Status

approved