A103048 - OEIS

A103048

Numbers k such that 7*10^k - 9 is prime.

1, 2, 3, 4, 14, 15, 23, 28, 54, 100, 272, 373, 403, 568, 639, 842, 969, 1255, 1259, 3047, 4838, 6389, 12755, 15142, 34943, 37652, 38108, 38686, 39384, 43393, 47280, 55030, 161192, 226479

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OFFSET

1,2

COMMENTS

Also numbers k such that 6*10^k + 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

a(35) > 3*10^5. - Robert Price, Jul 02 2023

LINKS

Table of n, a(n) for n=1..34.

Makoto Kamada, Prime numbers of the form 699...991.

Index entries for primes involving repunits.

FORMULA

a(n) = A101541(n) + 1.

MATHEMATICA

Do[ If[ PrimeQ[7*10^n - 9], Print[n]], {n, 0, 10000}]

Select[Range[100000], PrimeQ[(7 10^# - 9)] &] (* Vincenzo Librandi, Oct 15 2015 *)

PROG

(Magma) [n: n in [1..500] | IsPrime(7*10^n-9)]; // Vincenzo Librandi, Oct 15 2015

(PARI) isok(n) = isprime(7*10^n-9 ); \\ Michel Marcus, Oct 15 2015

CROSSREFS

Cf. A002275, A101541.

Sequence in context: A091907 A354075 A344313 * A343819 A140128 A167906

Adjacent sequences: A103045 A103046 A103047 * A103049 A103050 A103051

KEYWORD

more,nonn

AUTHOR

Robert G. Wilson v, Jan 18 2005

EXTENSIONS

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

Addition of a(25)-a(32) from Kamada data by Robert Price, Dec 13 2010

a(33) from Robert Price, Oct 14 2015

a(34) from Robert Price, Jul 02 2023

STATUS

approved