A046720 Subsequence of beastly primes (A186086) that are palindromes that begin and end with 7.

76667, 700666007, 700000666000007, 70000006660000007

Offset: 1

Author

G. L. Honaker, Jr.

Keywords

• base
• nonn

Comments

Next term is 7_{0}^48_666_{0}^48_7, containing 101 digits, and is too large to include here.

The number of digits in the terms is 2*A186521(n)+3: 5, 9, 15, 17, 101, 1159, 1589, 2647, 2787, 4787, 6135, 26961 (some correspond to probable primes). - Jens Kruse Andersen, Jul 13 2014

Links

• Jens Kruse Andersen, Table of n, a(n) for n = 1..7
• R. Ondrejka, The Top Ten: a Catalogue of Primal Configurations
• Tony Padilla and Brady Haran, The Most Evil Number, Numberphile video (2018)

Crossrefs

Cf. A051003, A131645, A186086, A186521.

Programs

Mathematica

Select[Table[(7*10^(n + 2) + 666)*10^n + 7, {n, 7}], PrimeQ] (* Arkadiusz Wesolowski, Sep 08 2011 *)
Select[Table[With[{s=PadRight[{7},n,0]},FromDigits[Join[s,{6,6,6},Reverse[s]]]],{n,8}],PrimeQ] (* Harvey P. Dale, Aug 05 2024 *)

Formula

a(n) = (7*10^(k+2)+666)*10^k+7, where k = A186521(n). - Jens Kruse Andersen, Jul 13 2014

Extensions

Definition revised by N. J. A. Sloane, Feb 14 2011