A256458 Numbers k such that 3*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 4, 5, 6, 11, 26, 92, 98, 110, 215, 232, 354, 522, 648, 1862, 2322, 2934, 3797, 6017, 11236, 24748, 30011, 37864, 42904, 57414, 195478

Offset

1,1

Comments

Also, numbers k such that (103*10^k - 1)/3 is prime.

Terms from Kamada.

a(27) > 250000.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Prime numbers of the form 3433...33.

Index entries for primes involving repunits.

Example

For k=4, 3*R_6 + 10^4 = 333333 + 10000 = 343333 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(103*10^#-1)/3 ] &]

Crossrefs

Cf. A002275.

Sequence in context: A194637 A194600 A255543 * A276001 A182109 A006539

Adjacent sequences: A256455 A256456 A256457 * A256459 A256460 A256461

Keyword

more,hard,nonn

Author

Robert Price, Apr 14 2015

Status

approved