A256925 Numbers k such that R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

136, 184, 640, 37960, 217360

Offset

1,1

Comments

Also, numbers k such that (109*10^k - 1)/9 is prime.

Terms from Kamada.

a(6) > 250000.

Numbers k such that 12 followed by k ones is prime. - Harvey P. Dale, Jan 30 2022

Links

Table of n, a(n) for n=1..5.

Makoto Kamada, Near-repdigit numbers of the form ABAA...AA.

Makoto Kamada, Prime numbers of the form 1211...11.

Index entries for primes involving repunits.

Mathematica

Select[Range[0, 250000], PrimeQ[(109*10^#-1)/9 ] &]
Select[Range[700], PrimeQ[FromDigits[PadRight[{1, 2}, #, 1]]]&]-2 (* The program generates the first 3 terms of the sequence. To generate more, increase the Range constant but the program may take a long time to run. *) (* Harvey P. Dale, Jan 30 2022 *)

Prog

(Magma) [n: n in [0..300] | IsPrime((109*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 700, if(isprime((109*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A072889 A157714 A165337 * A235285 A391530 A282794

Adjacent sequences: A256922 A256923 A256924 * A256926 A256927 A256928

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved