A056720 Numbers k such that 7*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 3, 5, 53, 56, 343, 908, 1079, 2204, 2379, 9134, 9371, 9728, 11039, 11247, 13275, 16781, 20138, 75368, 87863

Offset

1,3

Comments

Also numbers k such that (22*10^k - 1)/3 is prime.

a(23) > 10^5. - Robert Price, Dec 26 2014

Links

Table of n, a(n) for n=1..22.

Makoto Kamada, Prime numbers of the form 733...33.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 7*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

Crossrefs

Cf. A002275, A093675.

Sequence in context: A062608 A041791 A322947 * A100850 A309607 A060085

Adjacent sequences: A056717 A056718 A056719 * A056721 A056722 A056723

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(16)-a(22) from Robert Price, Dec 26 2014

Status

approved