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%I #33 Oct 12 2023 17:00:55
%S 0,1,7,13,58,129,253,1657,2244,2437,7924,9903,11899,18157,18957,23665,
%T 105609
%N Numbers k such that (7*10^(2k+1) - 18*10^k - 7)/9 is prime.
%C a(17) > 10^5. - _Robert Price_, Jun 23 2017
%C 0 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 with 0 occurrences of the digit 7. - _Robert Price_, Jun 23 2017
%D C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
%H Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp757">Palindromic Wing Primes (PWP's)</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77577.htm#prime">Prime numbers of the form 77...77577...77</a>
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = (A077785(n) - 1)/2.
%t Do[If[PrimeQ[(7*10^(2n + 1) - 18*10^n - 7)/9], Print[n]], {n, 3000}]
%o (PARI) is(n)=ispseudoprime((7*10^(2*n+1)-18*10^n-7)/9) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K nonn,base
%O 1,3
%A _Ray Chandler_, Dec 28 2010
%E a(16) from _Robert Price_, Jun 23 2017
%E a(17) from _Robert Price_, Oct 12 2023
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