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%I #30 Feb 13 2020 00:55:50
%S 2,3,6,23,36,69,561,723,3438,4104,9020,13977,19655,32400
%N Numbers n such that 7*(10^(2n+1)-1)/9 - 3*10^n is prime.
%C Original name: Numbers n such that (7*10^(2n+1)-27*10^n-7)/9 is prime.
%C a(15) > 10^5. - _Robert Price_, Nov 23 2015
%D 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 Patrick De Geest, World!Of Numbers, <a href="http://www.worldofnumbers.com/wing.htm#pwp747">Palindromic Wing Primes (PWP's)</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77477.htm#prime">Prime numbers of the form 77...77477...77</a>
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = (A077781(n)-1)/2.
%t Do[If[PrimeQ[(7*10^(2n + 1) - 27*10^n - 7)/9], Print[n]], {n, 3000}]
%o (PARI) for(n=1, 1e3, if(ispseudoprime((7*10^(2*n+1)-27*10^n-7)/9), print1(n, ", "))) \\ _Altug Alkan_, Nov 23 2015
%Y Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
%K nonn,base
%O 1,1
%A _Ray Chandler_, Dec 28 2010
%E a(14) from _Robert Price_, Nov 23 2015
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