A257034 Numbers k such that 7*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 3, 9, 16, 30, 45, 123, 171, 295, 324, 2785, 2791, 3910, 4015, 4050, 6319, 6415, 14670

Offset

1,3

Comments

Also, numbers k such that (718*10^k - 7)/9 is prime.

Terms from Kamada.

a(20) > 2*10^5.

Links

Table of n, a(n) for n=1..19.

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

Makoto Kamada, Prime numbers of the form 7977...77.

Index entries for primes involving repunits.

Example

For k=3, 7*R_5 + 2*10^3 = 77777 + 2000 = 79777 which is prime.

Mathematica

Select[Range[0, 30000], PrimeQ[(718*10^#-7)/9 ] &]

Prog

(Magma) [n: n in [0..400] | IsPrime((718*10^n-1) div 9)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A244475 A212870 A306447 * A232167 A058538 A197531

Adjacent sequences: A257031 A257032 A257033 * A257035 A257036 A257037

Keyword

nonn,hard,more

Author

Robert Price, Apr 14 2015

Status

approved