A256927 Numbers k such that R_(k+2) + 3*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

9, 26, 1268, 14391

Offset

1,1

Comments

Also, numbers k such that (127*10^k - 1)/9 is prime.

Terms from Kamada.

a(5) > 250000.

Links

Table of n, a(n) for n=1..4.

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

Makoto Kamada, Prime numbers of the form 1411...11.

Index entries for primes involving repunits.

Example

For k=9, R_11 + 3*10^9 = 11111111111 + 3000000000 = 14111111111 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(127*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((127*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 400, if(isprime((127*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A325585 A056409 A056399 * A135864 A074954 A103952

Adjacent sequences: A256924 A256925 A256926 * A256928 A256929 A256930

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved