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 A260866 #19 Jun 29 2019 11:27:25 %S A260866 0,1,289,74529,19088161,4886709025,1250999747361,320255971115809, %T A260866 81985529178309409,20988295478809805601,5373003642721911784225, %U A260866 1375488932539155041567521,352125166730061220638180129,90144042682896272963324429089,23076874926821455486290258903841 %N A260866 Base-16 representation of a(n) is the concatenation of the base-16 representations of 1, 2, ..., n, n-1, ..., 1. %C A260866 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=16, and c = R(b,b) = (b^n-1)/(b-1) is the base-b repunit of length b. %H A260866 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 A260866 For n < b = 16, 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 A260866 a(0) = 0 is the result of the empty sum corresponding to 0 digits. %e A260866 a(2) = (16+1)^2 = 16^2 + 2*16 + 1 = 121_16, concatenation of (1, 2, 1). %e A260866 a(17) = 123456789abcdef101110fedcba987654321_16 is the concatenation of (1, 2, 3, ..., 9, a, ..., f, 10, 11, 10, f, e, ..., 1), where the middle "10, 11, 10" are the base-16 representations of 16, 17, 16. %o A260866 (PARI) a(n,b=16)=sum(i=1,#n=concat(vector(n*2-1,k,digits(min(k,n*2-k),b))),n[i]*b^(#n-i)) %Y A260866 Base-16 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260865 for variants in other bases. %K A260866 nonn,base %O A260866 0,3 %A A260866 _M. F. Hasler_, Aug 01 2015