A077780 - OEIS

%I #31 Mar 26 2020 11:13:45

%S 5,7,65,91,3089

%N Numbers k such that (10^k - 1)/9 + 3*10^floor(k/2) is palindromic wing prime (a.k.a. near-repdigit palindromic prime).

%C Prime versus probable prime status and proofs are given in the author's table.

%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#pwp141">Palindromic Wing Primes (PWP's)</a>

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11411.htm#prime">Prime numbers of the form 11...11411...11</a>

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

%F a(n) = 2*A107124(n) + 1.

%e 7 is a term because (10^7 - 1)/9 + 3*10^3 = 1114111.

%t Do[ If[ PrimeQ[(10^n + 27*10^Floor[n/2] - 1)/9], Print[n]], {n, 3, 3100, 2}] (* _Robert G. Wilson v_, Dec 16 2005 *)

%Y Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.

%K more,nonn,base

%O 1,1

%A _Patrick De Geest_, Nov 16 2002

%E Name corrected by _Jon E. Schoenfield_, Oct 31 2018