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 A077787 #35 Jan 23 2025 21:52:44
%S A077787 21,29,81,119,321,825,1121,2579,3693
%N A077787 Numbers k such that (10^k - 1)/9 + 5*10^floor(k/2) is a palindromic wing prime (a.k.a. near-repdigit palindromic prime).
%C A077787 Prime versus probable prime status and proofs are given in the author's table.
%C A077787 a(10) > 4*10^5. - __Robert Price_, Jan 23 2025
%D A077787 C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
%H A077787 Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp161">Palindromic Wing Primes (PWP's)</a>
%H A077787 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11611.htm#prime">Prime numbers of the form 11...11611...11</a>
%H A077787 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A077787 a(n) = 2*A107126(n) + 1.
%e A077787 21 is a term because (10^21 - 1)/9 + 5*10^10 = 111111111161111111111.
%t A077787 Do[ If[ PrimeQ[(10^n + 45*10^Floor[n/2] - 1)/9], Print[n]], {n, 3, 4000, 2}] (* _Robert G. Wilson v_, Dec 16 2005 *)
%Y A077787 Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K A077787 more,nonn,base
%O A077787 1,1
%A A077787 _Patrick De Geest_, Nov 16 2002
%E A077787 Name corrected by _Jon E. Schoenfield_, Oct 31 2018