A089147 - OEIS

A089147

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

1, 3, 9, 13, 42, 51, 54, 91, 120, 168, 510, 819, 1071, 1756, 3010, 4333, 9534, 17839, 114846

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OFFSET

1,2

COMMENTS

Also numbers k such that (16*10^k - 7)/9 is prime.

a(19) > 10^5. - Robert Price, Dec 30 2014

a(20) > 150000. - Toshitaka Suzuki, Feb 13 2025

LINKS

MATHEMATICA

Do[ If[ PrimeQ[10^n + 7(10^n - 1)/9], Print[ n]], {n, 7000}]

CROSSREFS

Cf. A002275, A088465 (corresponding primes).

KEYWORD

base,nonn,more

AUTHOR

EXTENSIONS

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

a(18) from Robert Price, Dec 30 2014

a(19) from Toshitaka Suzuki, Feb 13 2025

STATUS

approved