A275522 Numbers k such that (28*10^k + 773)/9 is prime.

0, 2, 5, 6, 11, 20, 32, 59, 81, 98, 128, 437, 758, 989, 998, 1403, 1548, 1907, 1914, 2219, 5414, 9047, 13196, 18518, 28382

Offset

1,2

Comments

For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 1 followed by the digits 97 is prime (see Example section).

a(26) > 10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 31w97.

Example

5 is in this sequence because (28*10^5 + 773)/9 = 311197 is prime.
Initial terms and associated primes:
a(1) = 0, 89;
a(2) = 2, 397;
a(3) = 5, 311197;
a(4) = 6, 3111197;
a(5) = 11, 311111111197, etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(28*10^# + 773)/9] &]

Prog

(Magma) [n: n in [0..500] | IsPrime((28*10^n+773) div 9)]; // Vincenzo Librandi, Aug 01 2016
(PARI) is(n)=ispseudoprime((28*10^n+773)/9) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

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

Sequence in context: A135476 A255310 A051217 * A110975 A190120 A069789

Adjacent sequences: A275519 A275520 A275521 * A275523 A275524 A275525

Keyword

nonn,more,changed

Author

Robert Price, Jul 31 2016

Status

approved