A293033 Numbers k such that (7*10^k - 151)/9 is prime.

2, 3, 5, 6, 8, 11, 29, 99, 134, 407, 516, 986, 1482, 2709, 3326, 4395, 5277, 11712, 13067, 22542, 50414, 70616, 110006

Offset

1,1

Comments

For k>1, numbers such that k-2 occurrences of the digit 7 followed by the digits 61 is prime (see Example section).

a(24) > 2*10^5.

Links

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

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

Makoto Kamada, Search for 7w61

Example

3 is in this sequence because (7*10^3 - 151)/9 = 761 is prime.
Initial terms and primes associated:
a(1) = 2, 61;
a(2) = 3, 761;
a(3) = 5, 77761;
a(4) = 6, 777761;
a(5) = 8, 77777761; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[(7*10^# - 151)/9] &]

Crossrefs

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

Sequence in context: A107947 A120768 A271109 * A370708 A002384 A096176

Adjacent sequences: A293030 A293031 A293032 * A293034 A293035 A293036

Keyword

nonn,more,hard

Author

Robert Price, Sep 28 2017

Extensions

a(23) from Robert Price, Feb 14 2018

Status

approved