A183182 - OEIS

%I #30 Aug 03 2024 18:58:16

%S 1,3,39,54,168,240,5328,6159,24675,52227,113887

%N Numbers k such that (7*10^(2*k+1) + 9*10^k - 7)/9 is prime.

%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#pwp787">Palindromic Wing Primes (PWP's)</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77877.htm#prime">Prime numbers of the form 77...77877...77</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%F a(n) = (A077793(n) - 1)/2.

%t Do[If[PrimeQ[(7*10^(2n + 1) + 9*10^n - 7)/9], Print[n]], {n, 3000}]

%o (PARI) is(n)=ispseudoprime((7*10^(2*n+1)+9*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

%E a(9) from _Robert Price_, Oct 07 2023

%E a(10) from _Robert Price_, Oct 30 2023

%E a(11) from _Robert Price_, Aug 03 2024