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 A321687 #12 Nov 24 2018 01:34:39 %S A321687 1,121,12321,1234321,123454321,12345654321,1234567654321, %T A321687 123456787654321,12345678987654321,1234567898987654321, %U A321687 123456789878987654321,12345678987678987654321,1234567898765678987654321,123456789876545678987654321,12345678987654345678987654321 %N A321687 a(n) = (2*n+1)-digit palindrome formed by first concatenating A158289(i) for i=1..n+1 and then A158289(n-j) for j=1..n. %e A321687 1 %e A321687 121 %e A321687 12321 %e A321687 1234321 %e A321687 123454321 %e A321687 12345654321 %e A321687 1234567654321 %e A321687 123456787654321 %e A321687 12345678987654321 %e A321687 1234567898987654321 %e A321687 123456789878987654321 %e A321687 12345678987678987654321 %e A321687 1234567898765678987654321 %e A321687 123456789876545678987654321 %e A321687 12345678987654345678987654321 %e A321687 1234567898765432345678987654321 %e A321687 123456789876543212345678987654321 %e A321687 12345678987654321012345678987654321 %e A321687 1234567898765432101012345678987654321 %e A321687 123456789876543210121012345678987654321 %o A321687 (PARI) a158289(n)=abs(n-round(n/18)*18) %o A321687 eva(n) = subst(Pol(n), x, 10) %o A321687 a(n) = if(n==0, return(1)); my(v=vector(2*n+1), x=#v); for(k=1, ceil(#v/2), v[k]=a158289(k)); while(1, v[x]=v[#v-x+1]; x--; if(x==ceil(#v/2), return(eva(v)))) %Y A321687 Cf. A158289. %K A321687 nonn,base,easy %O A321687 0,2 %A A321687 _Felix Fröhlich_, Nov 17 2018