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 A028990 #15 Jan 25 2024 12:18:48 %S A028990 7,929,98689,9989899,999727999,99999199999,9999987899999, %T A028990 999999787999999,99999999299999999,9999999992999999999, %U A028990 999999999757999999999,99999999997579999999999 %N A028990 Largest palindromic prime with 2n-1 digits. %C A028990 For n>1, since 11 is a proper divisor of a palindrome with 2n digits, the numbers a(n) and A028989(n+1) are consecutive in A002385. - _Jeppe Stig Nielsen_, Feb 20 2021 %H A028990 Jeppe Stig Nielsen, <a href="/A028990/b028990.txt">Table of n, a(n) for n = 1..500</a> %H A028990 Patrick De Geest, <a href="http://www.worldofnumbers.com/palpri.htm">World!Of Palindromic Primes</a> %o A028990 (PARI) a(n)=forstep(j=10^n-1,10^(n-1),-1,my(d=digits(j), p=fromdigits(vector(2*n-1,x,if(x<n,d[x],d[2*n-x]))));ispseudoprime(p)&&return(p));0 \\ _Jeppe Stig Nielsen_, Feb 20 2021 %Y A028990 Cf. A002385, A028989. %K A028990 nonn,base %O A028990 1,1 %A A028990 _Patrick De Geest_ %E A028990 Offset changed (as it was in A028989) by _Jeppe Stig Nielsen_, Feb 20 2021