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 A260865 #18 Jun 29 2019 11:27:22 %S A260865 0,1,256,58081,13075456,2942086081,661970995456,148943498386081, %T A260865 33512287502995456,7540264693665886081,1696559556157202995456, %U A260865 381725900136606353386081,85888327530754964702995456,19324873694420145086040886081 %N A260865 Base-15 representation of a(n) is the concatenation of the base-15 representations of 1, 2, ..., n, n-1, ..., 1. %C A260865 See A260343 for the bases b such that B(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=15, and c = R(b,b) = (b^n-1)/(b-1) is the base-b repunit of length b. %H A260865 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 A260865 For n < b = 15, 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 A260865 a(0) = 0 is the result of the empty sum corresponding to 0 digits. %e A260865 a(2) = (15+1)^2 = 15^2 + 2*15 + 1 = 121_15, concatenation of (1, 2, 1). %e A260865 a(16) = 123456789abcde101110edcba987654321_15 is the concatenation of (1, 2, 3, ..., 9, a, ..., e, 10, 11, 10, e, d, ..., 1), where "e, 10, 11" are the base-15 representations of 14, 15, 16. %o A260865 (PARI) a(n,b=15)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i)) %Y A260865 Base-15 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases. %K A260865 nonn,base %O A260865 0,3 %A A260865 _M. F. Hasler_, Aug 01 2015