A102740 Numbers k such that 7*10^k - 11 is prime.

1, 6, 7, 9, 10, 11, 16, 42, 53, 78, 321, 699, 1858, 3425, 4899, 5734, 11081, 11675, 12136, 14056, 16074, 77969, 158465

Offset

1,2

Comments

a(24) > 2*10^5.

Numbers corresponding to terms <= 699 are certified primes. - Klaus Brockhaus, Feb 15 2005

For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 89 is prime (see Example section).

Links

Table of n, a(n) for n=1..23.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 69w89.

Example

Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 6, 6999989;
a(3) = 7, 69999989;
a(4) = 9, 6999999989;
a(5) = 10, 69999999989; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[7*10^# - 11] &]

Prog

(PARI) is(n)=ispseudoprime(7*10^n - 11) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Sequence in context: A131956 A205720 A348465 * A248178 A284094 A120183

Adjacent sequences: A102737 A102738 A102739 * A102741 A102742 A102743

Keyword

more,nonn

Author

Tom Mueller (muel4503(AT)uni-trier.de), Feb 08 2005

Extensions

a(11)-a(13) from Klaus Brockhaus, Feb 15 2005

a(14)-a(22) from Robert Price, Oct 29 2017

a(23) from Robert Price, Jun 21 2019

Status

approved