This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A260855 #13 Jun 29 2019 11:23:34 %S A260855 0,1,36,961,24336,3034961,1896581836,1185364159961,740852620019336, %T A260855 463032888020409961,289395555025471581836,180872221891237629784961, %U A260855 113045138682031465901269336,70653211676269864870442284961,44158257297668670511080159081836 %N A260855 Base-5 representation of a(n) is the concatenation of the base-5 representations of 1, 2, ..., n, n-1, ..., 1. %C A260855 Base-5 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases b = 3, ..., 16. %C A260855 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 A260855 D. Broadhurst, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;af419558.150">Primes from concatenation: results and heuristics</a>, NmbrThry List, August 1, 2015 %e A260855 a(0) = 0 is the result of the empty sum corresponding to 0 digits. %e A260855 a(2) = 36 = (5+1)^2 = 5^2 + 2*5 + 1 = 121_4 is the concatenation of (1, 2, 1). %e A260855 a(5) = 3034961 = 1234104321_5 is the concatenation of (1, 2, 3, 4, 10, 4, 3, 2, 1), where the middle "10" is the base-5 representation of 5. %o A260855 (PARI) a(n,b=5)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i)) %K A260855 nonn,base %O A260855 0,3 %A A260855 _M. F. Hasler_, Aug 01 2015