A256928 Numbers k such that R_(k+2) + 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 22, 280, 944, 1376, 1837, 2329, 2512, 5186, 24454, 25354, 49774, 103474, 159736, 177754

Offset

1,2

Comments

Also, numbers k such that (136*10^k - 1)/9 is prime.

Terms from Kamada.

a(17) > 250000.

Links

Table of n, a(n) for n=1..16.

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

Makoto Kamada, Prime numbers of the form 1511...11.

Index entries for primes involving repunits.

Example

For k=2, R_4 + 4*10^2 = 1111 + 400 = 1511 which is prime.

Mathematica

Select[Range[0, 250000], PrimeQ[(136*10^#-1)/9 ] &]

Prog

(Magma) [n: n in [0..300] | IsPrime((136*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((136*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

Crossrefs

Cf. A002275.

Sequence in context: A135634 A156505 A367979 * A349107 A368447 A299824

Adjacent sequences: A256925 A256926 A256927 * A256929 A256930 A256931

Keyword

more,hard,nonn

Author

Robert Price, Apr 13 2015

Status

approved