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 A077781 #44 Mar 26 2020 06:38:15
%S A077781 5,7,13,47,73,139,1123,1447,6877,8209,18041,27955,39311,64801
%N A077781 Numbers k such that 7*(10^k - 1)/9 - 3*10^floor(k/2) is a palindromic wing prime (also known as near-repdigit palindromic prime).
%C A077781 Prime versus probable prime status and proofs are given in the author's table.
%C A077781 a(15) > 2*10^5. - _Robert Price_, Nov 23 2015
%D A077781 C. Caldwell and H. Dubner, The near repdigit primes A(n-k-1)B(1)A(k), especially 9(n-k-1)8(1)9(k), Journal of Recreational Mathematics, Volume 28, No. 1, 1996-97, pp. 1-9.
%H A077781 Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp747">Palindromic Wing Primes (PWP's)</a>
%H A077781 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77477.htm#prime">Prime numbers of the form 77...77477...77</a>
%H A077781 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A077781 a(n) = 2*A183179(n) + 1.
%e A077781 7 is a term because 7*(10^7 - 1)/9 - 3*10^3 = 7774777.
%t A077781 Do[ If[ PrimeQ[(7*10^n - 27*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 39400, 2}] (* _Robert G. Wilson v_, Dec 16 2005 *)
%o A077781 (PARI) for(n=1, 1e3, if(ispseudoprime((7*10^(2*n+1)-27*10^n-7)/9), print1(2*n+1, ", "))) \\ _Altug Alkan_, Nov 23 2015
%Y A077781 Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K A077781 more,nonn,base
%O A077781 1,1
%A A077781 _Patrick De Geest_, Nov 16 2002
%E A077781 a(14) from _Robert Price_, Nov 23 2015
%E A077781 Name corrected by _Jon E. Schoenfield_, Oct 31 2018