OFFSET
1,2
COMMENTS
By definition, all terms are palindromes. Inspired by A261493.
There are no primes in this sequence up to a(1100).
The least prime factors of a(n), n>=1, are: 1, 11, 3, 11, 41, 3, 239, 11, 3, 11, 11, 3, 11, 11, 3, 11, 11, 3, 71, 21557, 19, 17, 31, 181, 17, 353, 19, 31, 19, 29, 17, 29, 11616377, 214141, 19, 5471, 17, 13883, 3, 7, ..., . See A261411.
The first (probable) prime in this sequence was found by David Broadhurst on Aug 25 2015: this is a(2007), a 21233-digit probable prime with central term 1008001. - N. J. A. Sloane, Aug 24 2015
LINKS
M. F. Hasler, Table of n, a(n) for n = 1..108
EXAMPLE
a(4) is the concatenation of 1, 2, 3 and 4, and then 3, 2 and 1 which results in 1234321.
MATHEMATICA
palQ[n_] := Reverse[idn = IntegerDigits@ n] == idn; s = Select[ Range @111, palQ]; f[n_] := FromDigits@ Flatten[ IntegerDigits@# & /@ Join[Take[s, n], Reverse@ Take[s, n - 1]]]; a = Array[f, 14]
PROG
CROSSREFS
KEYWORD
nonn,easy,base
AUTHOR
Robert G. Wilson v, Aug 24 2015
STATUS
approved