A056704 Numbers k such that 3*10^k + 1*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 5, 10, 11, 13, 34, 47, 52, 77, 88, 554, 580, 1310, 1505, 8537, 15892, 24022, 27041, 37922, 40033, 134122, 165358, 183760

Offset

1,3

Comments

Also numbers k such that (28*10^k - 1)/9 is prime.

Although perhaps a degenerate case, A002275 defines R(0)=0. Thus zero belongs in this sequence since 3*10^0 + 0 = 3*1 + 0 = 3 is prime. - Robert Price, Oct 28 2014

a(26) > 2*10^5. - Robert Price, Dec 19 2014

Links

Table of n, a(n) for n=1..25.

Makoto Kamada, Prime numbers of the form 311...11.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 3*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A068813.

Sequence in context: A136817 A140180 A175324 * A230408 A163624 A353356

Adjacent sequences: A056701 A056702 A056703 * A056705 A056706 A056707

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

Added zero by Robert Price, Oct 28 2014

a(18)-a(25) from Kamada data by Robert Price, Dec 19 2014

Status

approved