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 A077785 #41 Oct 12 2023 14:10:40
%S A077785 3,15,27,117,259,507,3315,4489,4875,15849,19807,23799,36315,37915,
%T A077785 47331,211219
%N A077785 Odd numbers k such that the palindromic wing number (a.k.a. near-repdigit palindrome) 7*(10^k - 1)/9 - 2*10^((k-1)/2) is prime.
%C A077785 Original name was "Palindromic wing primes (a.k.a. near-repdigit palindromes) of the form 7*(10^a(n)-1)/9-2*10^[ a(n)/2 ]."
%C A077785 Prime versus probable prime status and proofs are given in the author's table.
%C A077785 a(16) > 2*10^5. - _Robert Price_, Jun 23 2017
%C A077785 1 could be considered part of this sequence since the formula evaluates to 5 which is a degenerate form of the near-repdigit palindrome 777...77577...777 that has zero occurrences of the digit 7. - _Robert Price_, Jun 23 2017
%D A077785 C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
%H A077785 Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp757">Palindromic Wing Primes (PWP's)</a>
%H A077785 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77577.htm#prime">Prime numbers of the form 77...77577...77</a>
%H A077785 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A077785 a(n) = 2*A183180(n) + 1.
%e A077785 15 is in the sequence because 7*(10^15 - 1)/9 - 2*10^7 = 777777757777777 is prime.
%t A077785 Do[ If[ PrimeQ[(7*10^n - 18*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 40000, 2}] (* _Robert G. Wilson v_, Dec 16 2005 *)
%Y A077785 Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K A077785 more,nonn,base
%O A077785 1,1
%A A077785 _Patrick De Geest_, Nov 16 2002
%E A077785 a(15) from _Robert Price_, Jun 23 2017
%E A077785 Example edited by _Jon E. Schoenfield_, Jun 23 2017
%E A077785 Name edited by _Jon E. Schoenfield_, Jun 24 2017
%E A077785 a(16) from _Robert Price_, Oct 12 2023