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 A077775 #42 Mar 05 2019 07:58:50
%S A077775 3,7,15,123,181,185,539,597,643,743,1553,3135,4769,5133,6177,11733,
%T A077775 16103,18997,25271,49025,65043,87965
%N A077775 Odd numbers k such that (10^k - 1)/3 - 2*10^floor(k/2) is a palindromic wing prime (a.k.a. near-repdigit palindromic prime) of the form 3...313...3.
%C A077775 Prime versus probable prime status and proofs are given in the author's table.
%C A077775 a(23) > 2*10^5. - _Robert Price_, Jan 29 2016
%C A077775 Primes of the form (10^k-1)/3 - 2*10^floor(k/2) are obtained for k in (2, 3, 6, 7, 8, 10, 15, 22, 34, 123, 126, 144, 181, 185, 198, 534, 539, 597, 606, ...). For example (10^2 - 1)/3 - 2*10^1 = 13 is also prime. However, for even k the result is not palindromic. See A077775-A077798, A107123-A107127 for PWP's with digits other than 3 and 1. - _M. F. Hasler_, Mar 03 2019
%D A077775 C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
%H A077775 Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp313">Palindromic Wing Primes (PWP's)</a>
%H A077775 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33133.htm#prime">Prime numbers of the form 33...33133...33</a>
%H A077775 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A077775 a(n) = 2*A183174(n) + 1.
%e A077775 a(3) = 15 corresponds to the prime (10^15 - 1)/3 - 2*10^7 = 333333313333333.
%t A077775 Do[ If[ PrimeQ[(10^n - 6*10^Floor[n/2] - 1)/3], Print[n]], {n, 3, 49100, 2}] (* _Robert G. Wilson v_, Dec 16 2005 *)
%o A077775 (PARI) is(n)=bittest(n,0)&&ispseudoprime(10^n\3-2*10^(n\2)) \\ _M. F. Hasler_, Mar 03 2019
%Y A077775 Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K A077775 more,nonn,base
%O A077775 1,1
%A A077775 _Patrick De Geest_, Nov 16 2002
%E A077775 a(21)-a(22) from _Robert Price_, Jan 29 2016
%E A077775 a(21) corrected by _Robert Price_, Feb 03 2016
%E A077775 Name corrected by _Jon E. Schoenfield_, Oct 31 2018
%E A077775 Name edited by _M. F. Hasler_, Mar 03 2019