%I
%S 0,1,36,961,24336,3034961,1896581836,1185364159961,740852620019336,
%T 463032888020409961,289395555025471581836,180872221891237629784961,
%U 113045138682031465901269336,70653211676269864870442284961,44158257297668670511080159081836
%N Base5 representation of a(n) is the concatenation of the base5 representations of 1, 2, ..., n, n1, ..., 1.
%C Base5 variant of A173426 (base 10) and A173427 (base 2). See A260853  A260866 for variants in other bases b = 3, ..., 16.
%C The base 5 is not listed in A260343, because a(5) = A260851(5) = 3034961 is not prime and therefore not in A260852. See these sequences for more information.
%H D. Broadhurst, <a href="https://listserv.nodak.edu/cgibin/wa.exe?A2=NMBRTHRY;af419558.150">Primes from concatenation: results and heuristics</a>, NmbrThry List, August 1, 2015
%e a(0) = 0 is the result of the empty sum corresponding to 0 digits.
%e a(2) = 36 = (5+1)^2 = 5^2 + 2*5 + 1 = 121_4 is the concatenation of (1, 2, 1).
%e a(5) = 3034961 = 1234104321_5 is the concatenation of (1, 2, 3, 4, 10, 4, 3, 2, 1), where the middle "10" is the base5 representation of 5.
%o (PARI) a(n,b=5)=sum(i=1,#n=concat(vector(n*21,k,digits(min(k,n*2k),b))),n[i]*b^(#ni))
%K nonn,base
%O 0,3
%A _M. F. Hasler_, Aug 01 2015
