

A260851


a(n) in base n is the concatenation of the base n expansions of (1, 2, 3, ..., n1, n, n1, ..., 3, 2, 1).


14



1, 13, 439, 27961, 3034961, 522134761, 131870760799, 45954960939217, 21107054541321649, 12345678910987654321, 8954302429379707945271, 7883984846509322664831433, 8281481197999449959084458465, 10228838696316240496325238416281, 14674825961700306151086890240104831
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,2


COMMENTS

Sequences A173427, A260853  A260859, A173426, A260861  A260866, A260860 list the numbers A_b(n) whose base b expansion is the concatenation of the base b expansions of (1, 2, ..., n, n1, ..., 1). For n < b these are the squares of the repdigits of length n in base b, so the first candidate for a prime is the term with n = b. These are the numbers listed here. Sequence A260343 gives the bases b for which this is indeed a prime, the corresponding primes a(A260343(n)) are listed in A260852.
The initial term a(1) = 1 refers to the unary or "tally mark" representation of the numbers, cf. A000042. It can be considered as purely conventional.


LINKS

N. J. A. Sloane, Table of n, a(n) for n = 1..100


FORMULA

a(n) = n*r + (r  n)*(1 + n*r) = (r  n + 1)*(1 + n*r)  1, where r = (n^n1)/(n1) is the base n repunit of length n, r = 1 for n = 1.
Another closedform expression for the series is a(n) = (n^(2*n+1) + (n^3 + 2*n^2  2*n  1)*n^n + 1)/(n  1)^2.  Serge Batalov, Aug 02 2015


EXAMPLE

a(1) = 1 is the "concatenation" of (1) which is the unary representation of 1, cf A000042.
a(2) = 13 = 1101[2] = concatenation of (1, 10, 1), where 10 is the base 2 representation of 2.
a(3) = 439 = 121021[3] = concatenation of (1, 2, 10, 2, 1), where 10 is the base 3 representation of 3.
a(10) = 12345678910987654321 is the concatenation of (1, 2, 3, ..., 9, 10, 9, 8, ..., 2, 1); it is also a prime.


MAPLE

f:=proc(b) local i;
add((i+1)*b^i, i=0..b2) + b^b + add(i*b^(2*bi), i=1..b1); end;
[seq(f(b), b=1..25)]; # N. J. A. Sloane, Sep 26 2015


MATHEMATICA

Join[{1}, Table[((n^n  1)/(n  1)  n + 1) (1 + n (n^n  1)/(n  1))  1, {n, 2, 30}]] (* Vincenzo Librandi, Aug 02 2015 *)


PROG

(PARI) A260851(n)=(1+n*r=if(n>2, n^n\(n1), n*21))*(rn+1)1
(MAGMA) [1] cat [((n^n1)/(n1)  n + 1)*(1 + n*(n^n1)/(n1))  1: n in [2..15]]; // Vincenzo Librandi, Aug 02 2015


CROSSREFS

Cf. A173427, A260853  A260859, A173426, A260861  A260866, A260860.
For primes in this sequence see A260343, A260852.
Sequence in context: A012084 A114759 A260871 * A260852 A012832 A102075
Adjacent sequences: A260848 A260849 A260850 * A260852 A260853 A260854


KEYWORD

nonn,base


AUTHOR

M. F. Hasler, Aug 01 2015


STATUS

approved



