A046720 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

A046720

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

76667, 700666007, 700000666000007, 70000006660000007

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

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)

FORMULA

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

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 *)

CROSSREFS

Cf. A051003, A131645, A186086, A186521.

Sequence in context: A236709 A104327 A357608 * A258606 A152498 A237747