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%I #21 Jan 17 2019 13:44:08
%S 1,2,8,19,20,212,280,887,1021,5515,8116,11852
%N Numbers n such that (7*10^(2n+1)+18*10^n-7)/9 is prime.
%C a(13) > 10^5. - _Robert Price_, Jan 19 2016
%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#pwp797">Palindromic Wing Primes (PWP's)</a>
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77977.htm#prime">Prime numbers of the form 77...77977...77</a>
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = (A077796(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,more,base
%O 1,2
%A _Ray Chandler_, Dec 28 2010
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