A090149 -id:A090149 - cp's OEIS frontend

A055558 Primes of the form 1999...999.

19, 199, 1999, 199999, 19999999, 199999999999999999999999999, 1999999999999999999999999999, 199999999999999999999999999999999999999999999999999999

Offset: 1

Labos Elemer, Jul 10 2000

Primes of the form 2*10^k - 1.

			2*10^n - 1 is prime for {1,2,3,5,7,26,27,53,147,236,248,386,401}; in each of these numbers, the digit '9' appears n times.

Subsequence of A090149.
Primes in A067272.
Cf. A002957, A051200.

for(n=1,99,if(ispseudoprime(t=2*10^n-1),print1(t", "))) \\ Charles R Greathouse IV, Dec 10 2013

a(n) = 2*10^A002957(n) - 1 = A067272(A002957(n) + 1). - Elmo R. Oliveira, Jun 14 2025

A090157 Primes of the form identical digits preceded by a 9.

Original entry on oeis.org

97, 911, 977, 97777, 911111, 97777777777777777777, 911111111111111111111, 97777777777777777777777777777, 911111111111111111111111111111111111111111

Offset: 1

Robert G. Wilson v, Nov 22 2003

The identical digits must be either 1 or 7. - Harvey P. Dale, Jun 28 2020

Vincenzo Librandi, Table of n, a(n) for n = 1..21

Cf. A090148, A090149, A090150, A090151, A090152, A090153, A090154, A090155, A090157.

Select[ FromDigits /@ Flatten[ Table[ PadRight[{9}, i, # ] & /@ {1, 2, 3, 4, 5, 6, 7, 8, 9}, {i, 2, 50}], 1], PrimeQ[ # ] &]
Select[Union[Flatten[Table[FromDigits[PadRight[{9},n,d]],{n,2,50},{d,{1,7}}]]],PrimeQ]  (* Harvey P. Dale, Jun 27 2020 *)

cp's OEIS Frontend

A055558 Primes of the form 1999...999.

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