A257039 Numbers k such that 9*R_(k+2) - 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

10, 12, 63, 69, 156, 328, 340, 344, 444, 672, 894, 1464, 1670, 1708, 2010, 4306, 7888, 8864, 9478, 9621, 26004, 36992, 71600, 98738, 118949, 130565, 140326, 183452, 211983, 225618

Offset

1,1

Comments

Also, numbers k such that 96*10^k - 1 is prime.

Terms from Kamada.

846519 reported to Kamada by Bruno DallOsto is also in this sequence. It may or may not be a(31).

a(31) > 230000.

Links

Table of n, a(n) for n=1..30.

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

Makoto Kamada, Prime numbers of the form 9599...99.

Index entries for primes involving repunits.

Example

For k=10, 9*R_12 - 4*10^10 = 999999999999 - 40000000000 = 959999999999 which is prime.

Mathematica

Select[Range[0, 230000], PrimeQ[96*10^#-1 ] &]

Prog

(Magma) [n: n in [0..400] | IsPrime(96*10^n-1)]; // Vincenzo Librandi, Apr 15 2015

Crossrefs

Cf. A002275.

Sequence in context: A367149 A363285 A219917 * A337076 A223150 A333258

Adjacent sequences: A257036 A257037 A257038 * A257040 A257041 A257042

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved