A260859 - OEIS

A260859

Base-9 representation of a(n) is the concatenation of the base-9 representations of 1, 2, ..., n, n-1, ..., 1.

0, 1, 100, 8281, 672400, 54479161, 4412944900, 357449732641, 28953439105600, 21107054541321649, 138483384602892402628, 908589486379899193778809, 5961255620138564686107812272, 39111798123729126657669459066697, 256612507489786800304910707633347364

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,3

COMMENTS

The base 9 is listed in A260343, because a(9) = A260851(9) = 21107054541321649 = 123456781087654321_9 is prime and therefore in A260852. See these sequences for more information.

LINKS

Table of n, a(n) for n=0..14.

D. Broadhurst, Primes from concatenation: results and heuristics, NmbrThry List, August 1, 2015

FORMULA

For n < b = 9, we have a(n) = A_b(n) = R(b,n)^2, where R(b,n) = (b^n-1)/(b-1) are the base-b repunits.

EXAMPLE

a(0) = 0 is the result of the empty sum corresponding to 0 digits.

a(2) = 100 = (9+1)^2 = 9^2 + 2*9 + 1 = 121_9, concatenation of (1, 2, 1).

a(10) = 1234567810111087654321_9 is the concatenation of (1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 10, 8, 7, 6, 5, 4, 3, 2, 1), where the middle "10, 11, 10" are the base-9 representations of 9, 10, 9.

PROG

(PARI) a(n, b=9)=sum(i=1, #n=concat(vector(n*2-1, k, digits(min(k, n*2-k), b))), n[i]*b^(#n-i))

CROSSREFS

Base-9 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for the variants in other bases.

Sequence in context: A245666 A210814 A065689 * A117687 A262806 A108741

Adjacent sequences: A260856 A260857 A260858 * A260860 A260861 A260862

KEYWORD

nonn,base

AUTHOR

M. F. Hasler, Aug 01 2015

STATUS

approved