%I
%S 0,1,100,8281,672400,54479161,4412944900,357449732641,28953439105600,
%T 21107054541321649,138483384602892402628,908589486379899193778809,
%U 5961255620138564686107812272,39111798123729126657669459066697,256612507489786800304910707633347364
%N Base9 representation of a(n) is the concatenation of the base9 representations of 1, 2, ..., n, n1, ..., 1.
%C 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.
%H D. Broadhurst, <a href="https://listserv.nodak.edu/cgibin/wa.exe?A2=NMBRTHRY;af419558.1508">Primes from concatenation: results and heuristics</a>, NmbrThry List, August 1, 2015
%F For n < b = 9, we have a(n) = A_b(n) = R(b,n)^2, where R(b,n) = (b^n1)/(b1) are the baseb repunits.
%e a(0) = 0 is the result of the empty sum corresponding to 0 digits.
%e a(2) = 100 = (9+1)^2 = 9^2 + 2*9 + 1 = 121_9, concatenation of (1, 2, 1).
%e 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 base9 representations of 9, 10, 9.
%o (PARI) a(n,b=9)=sum(i=1,#n=concat(vector(n*21,k,digits(min(k,n*2k),b))),n[i]*b^(#ni))
%Y Base9 variant of A173426 (base 10) and A173427 (base 2). See A260853  A260866 for the variants in other bases.
%K nonn,base
%O 0,3
%A _M. F. Hasler_, Aug 01 2015
