A256931 Numbers k such that R_(k+2) + 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3, 16, 24, 165, 270, 478, 1683, 3976, 41608, 71760, 86982, 114226, 116610, 201504

Offset

1,1

Comments

Also, numbers k such that (145*10^k - 1)/9 is prime.

Terms from Kamada.

a(15) > 250000.

Links

Table of n, a(n) for n=1..14.

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

Makoto Kamada, Prime numbers of the form 1611...11.

Index entries for primes involving repunits.

Example

For k=3, R_5 + 5*10^2 = 11111 + 5000 = 16111 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(145*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((145*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 500, if(isprime((145*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A101132 A153723 A091273 * A101405 A335308 A193367

Adjacent sequences: A256928 A256929 A256930 * A256932 A256933 A256934

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved