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%I #31 Oct 10 2021 05:56:24
%S 0,1,3,7,10,12,480,949,1945,7548,8923
%N Numbers k such that 7*(10^(2*k+1)-1)/9 - 5*10^k is prime.
%C Original name: Numbers n such that (7*10^(2n+1)-45*10^n-7)/9 is prime.
%C a(12) > 10^5. - _Robert Price_, Nov 02 2015
%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#pwp727">Palindromic Wing Primes (PWP's)</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77277.htm#prime">Prime numbers of the form 77...77277...77</a>
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = (A077777(n-1)-1)/2 for n>1.
%t Do[If[PrimeQ[(7*10^(2n + 1) - 45*10^n - 7)/9], Print[n]], {n, 3000}]
%o (PARI) is(n)=ispseudoprime((7*10^(2*n+1)-45*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 Name edited and a(1) = 0 inserted by _M. F. Hasler_, Feb 07 2020
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