A256933 Numbers k such that R_(k+2) + 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 11, 62, 157, 170, 251, 500, 2275, 7525, 12230, 13658, 54727, 183227, 212779

Offset

1,2

Comments

Also, numbers k such that (163*10^k - 1)/9 is prime.

Terms from Kamada.

a(16) > 250000.

Links

Table of n, a(n) for n=1..15.

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

Makoto Kamada, Prime numbers of the form 1811...11.

Index entries for primes involving repunits.

Example

For k=2, R_4 + 7*10^2 = 1111 + 700 = 1811 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(163*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((163*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((163*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A126034 A308729 A034726 * A162274 A183160 A020078

Adjacent sequences: A256930 A256931 A256932 * A256934 A256935 A256936

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved