A256932 Numbers k such that R_(k+2) + 6*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 8, 9, 23, 54, 72, 92, 402, 405, 651, 860, 897, 2691, 3887, 4665, 4971, 5271, 8931, 9953, 10566, 10907, 11955, 22467, 66755, 153810, 169337, 224061

Offset

1,2

Comments

Also, numbers k such that (154*10^k - 1)/9 is prime.

Terms from Kamada.

a(28) > 250000.

Links

Table of n, a(n) for n=1..27.

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

Makoto Kamada, Prime numbers of the form 1711...11.

Index entries for primes involving repunits.

Example

For k=8, R_10 + 6*10^8 = 1111111111 + 600000000 = 1711111111 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(154*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((154*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((154*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A303957 A046258 A074344 * A304600 A278076 A081351

Adjacent sequences: A256929 A256930 A256931 * A256933 A256934 A256935

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved