A281493 Numbers k such that (10^k + 41)/3 is prime.

1, 2, 3, 4, 5, 8, 11, 13, 19, 46, 70, 98, 152, 187, 364, 425, 601, 665, 1837, 2267, 7691, 10246, 19988, 54290, 58571, 181996

Offset

1,2

Comments

For k > 1, numbers k such that k-2 occurrences of the digit 3 followed by the digits 47 is prime (see Example section).

a(27) > 2*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 3w47.

Example

3 is in this sequence because (10^3 + 41)/3 = 347 is prime.
Initial terms and associated primes:
a(1) = 1, 17;
a(2) = 2, 47;
a(3) = 3, 347;
a(4) = 4, 3347;
a(5) = 5, 33347; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(10^# + 41)/3] &]

Crossrefs

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

Sequence in context: A322531 A302591 A269571 * A181341 A174181 A298424

Adjacent sequences: A281490 A281491 A281492 * A281494 A281495 A281496

Keyword

nonn,more,hard

Author

Robert Price, Jan 22 2017

Extensions

a(26) from Robert Price, Dec 27 2017

Status

approved