A198973 -id:A198973 - cp's OEIS frontend

A093947 Primes of the form 8*10^k - 1.

7, 79, 79999, 799999, 799999999, 79999999999, 79999999999999999999999999, 79999999999999999999999999999999999999999999999999, 79999999999999999999999999999999999999999999999999999999999999999999999999999

Offset: 1

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Rick L. Shepherd, Apr 17 2004

nonn

Equivalently, primes of the form 7*10^k + 9*R_k, where R_k is the repunit (A002275) of length k.

Makoto Kamada, Prime numbers of the form 799...99.
Index entries for primes involving repunits.

Cf. A002275, A056721 (corresponding k).

Primes in A198973.

[a: n in [0..200] | IsPrime(a) where a is (8*10^n-1)]; // Vincenzo Librandi, May 08 2019

Select[Table[FromDigits[PadRight[{7}, n, 9]], {n, 50}], PrimeQ] (* Vincenzo Librandi, May 08 2019 *)

a(n) = 8*10^A056721(n) - 1 = A198973(A056721(n)). - Elmo R. Oliveira, Jun 14 2025

cp's OEIS Frontend

A093947 Primes of the form 8*10^k - 1.

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