A283513 Numbers k such that (85*10^k + 473)/9 is prime.

2, 3, 5, 6, 8, 9, 32, 56, 63, 80, 117, 185, 590, 777, 1962, 2654, 3341, 5484, 10256, 11051, 11816, 20622, 44750, 94802, 105242, 117719, 140609

Offset

1,1

Comments

For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 4 followed by the digits 97 is prime (see Example section).

a(28) > 2*10^5.

Links

Table of n, a(n) for n=1..27.

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

Makoto Kamada, Search for 94w97.

Example

3 is in this sequence because (85*10^3 + 473)/9 = 9497 is prime.
Initial terms and associated primes:
a(1) = 2, 997;
a(2) = 3, 9497;
a(3) = 5, 944497;
a(4) = 6, 9444497;
a(5) = 8, 944444497; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[ (85*10^# + 473)/9 ] &]

Prog

(PARI) isok(n) = isprime((85*10^n + 473)/9); \\ Indranil Ghosh, Mar 09 2017

Crossrefs

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

Sequence in context: A294941 A028374 A050578 * A028776 A028734 A028755

Adjacent sequences: A283510 A283511 A283512 * A283514 A283515 A283516

Keyword

nonn,more,hard,changed

Author

Robert Price, Mar 09 2017

Extensions

a(25)-a(27) from Robert Price, Jan 06 2020

Status

approved