A256926 Numbers k such that R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 4, 7, 16, 19, 37, 67, 166, 292, 20941

Offset

1,3

Comments

Also, numbers k such that (118*10^k - 1)/9 is prime.

Terms from Kamada.

a(12) > 250000.

Links

Table of n, a(n) for n=1..11.

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

Makoto Kamada, Prime numbers of the form 1311...11.

Index entries for primes involving repunits.

Example

For k=4, R_6 + 2*10^4 = 111111 + 20000 = 131111 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(118*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((118*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((118*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A212062 A353259 A345429 * A101534 A110933 A299423

Adjacent sequences: A256923 A256924 A256925 * A256927 A256928 A256929

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved