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 A260863 #11 Jun 29 2019 11:26:58 %S A260863 0,1,196,33489,5664400,957345481,161792190756,27342890695849, %T A260863 4620948663553600,780940325907974961,131978915101424183716, %U A260863 22304436652439380447009,3769449794266138309731600,8281481197999449959084458465,236527384496061684935031509169004 %N A260863 Base-13 representation of a(n) is the concatenation of the base-13 representations of 1, 2, ..., n, n-1, ..., 1. %C A260863 See A260343 for the bases b such that A260851(b) = A_b(b) = b*c + (c - b)*(1 + b*c), is prime, where A_b is the base-b sequence, as here with b = 13, and c = R(b,b) = (b^n-1)/(b-1) is the base-b repunit of length b. %H A260863 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 %F A260863 For n < b = 13, 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. %e A260863 a(0) = 0 is the result of the empty sum corresponding to 0 digits. %e A260863 a(2) = (13+1)^2 = 13^2 + 2*13 + 1 = 121_13, concatenation of (1, 2, 1). %e A260863 a(14) = 123456789abc101110cba987654321_13 is the concatenation of (1, 2, 3, ..., 9, a, b, c, 10, 11, 10, c, ..., 1), where "c, 10, 11" are the base-13 representations of 12, 13, 14. %o A260863 (PARI) a(n,b=13)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i)) %Y A260863 Base-13 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases. %K A260863 nonn,base %O A260863 0,3 %A A260863 _M. F. Hasler_, Aug 01 2015