A257029 Numbers k such that 7*R_(k+2) - 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 66, 84, 3366, 14934

Offset

1,2

Comments

Also, numbers k such that (664*10^k - 7)/9 is prime.

Terms from Kamada.

a(6) > 30000.

Links

Table of n, a(n) for n=1..5.

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

Makoto Kamada, Prime numbers of the form 7377...77.

Index entries for primes involving repunits.

Mathematica

Select[Range[0, 30000], PrimeQ[(664*10^#-7)/9 ] &]

Prog

(PARI) for(n=0, 100, if(isprime((664*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..400] | IsPrime((664*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A127654 A293175 A032485 * A223734 A031411 A098775

Adjacent sequences: A257026 A257027 A257028 * A257030 A257031 A257032

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved