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 A260856 #13 Sep 10 2018 15:08:11 %S A260856 0,1,49,1849,67081,2418025,522134761,676678989289,876975982612969, %T A260856 1136560874204496361,1472982892995886760425,1908985829323636470956521, %U A260856 2474045634803467686907986409,3206363142705295375772778742249,4155446632946062852128962559066601 %N A260856 Base-6 representation is the concatenation of the base-6 representations of 1, 2, ..., n, n-1, ..., 1. %H A260856 D. Broadhurst, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;af419558.1508">Primes from concatenation: results and heuristics</a>, NmbrThry List, August 1, 2015 %e A260856 a(0) = 0 is the result of the empty sum corresponding to 0 digits. %e A260856 a(2) = 49 = (6+1)^2 = 6^2 + 2*6 + 1 = 121_6 is the concatenation of (1, 2, 1). %e A260856 a(7) = 676678989289 = 1234510111054321_6 is the concatenation of (1, 2, 3, 4, 5, 10, 11, 10, 5, 4, 3, 2, 1), where the middle "10, 11, 10" are the base 6 representations of 6, 7, 6. %o A260856 (PARI) a(n,b=6)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i)) %Y A260856 Base 6 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases b = 3, ..., 16. %K A260856 nonn,base %O A260856 0,3 %A A260856 _M. F. Hasler_, Aug 01 2015