A283503 Numbers k such that (23*10^k - 59)/9 is prime.

3, 6, 10, 16, 22, 30, 33, 48, 61, 66, 147, 300, 372, 925, 1750, 2026, 2721, 3916, 5704, 6856, 12883, 57333, 92244, 147649, 150763

Offset

1,1

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 5 followed by the digits 49 is prime (see Example section).

a(26) > 2*10^5.

Links

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

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

Makoto Kamada, Search for 25w49.

Example

6 is in this sequence because (23*10^6 - 59)/9 =  is prime.
Initial terms and associated primes:
a(1) = 3, 2549;
a(2) = 6, 2555549;
a(3) = 10, 25555555549;
a(4) = 16, 25555555555555549;
a(5) = 22, 25555555555555555555549; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(23*10^# - 59)/9] &]

Prog

(PARI) isok(n) = isprime((23*10^n - 59)/9); \\ Indranil Ghosh, Mar 09 2017

Crossrefs

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

Sequence in context: A310084 A025215 A327141 * A294481 A049697 A111712

Adjacent sequences: A283500 A283501 A283502 * A283504 A283505 A283506

Keyword

nonn,more,hard

Author

Robert Price, Mar 09 2017

Extensions

a(24)-a(25) from Robert Price, May 29 2018

Status

approved