A284191 Numbers k such that 4*10^k - 87 is prime.

2, 5, 7, 13, 16, 20, 29, 37, 43, 49, 101, 888, 2533, 2569, 2599, 2666, 3689, 6335, 9634, 12445, 17669, 51208, 79729

Offset

1,1

Comments

For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 13 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 39w13.

Example

5 is in this sequence because 4*10^5 - 87 = 399913 is prime.
Initial terms and associated primes:
a(1) = 2, 313;
a(2) = 5, 399913;
a(3) = 7, 39999913;
a(4) = 13, 39999999999913;
a(5) = 16, 39999999999999913; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[4*10^#-87] &]

Prog

(PARI) isok(n) = isprime(4*10^n - 87); \\ Indranil Ghosh, Mar 22 2017
(Python)
from sympy import isprime
def ok(n): return 1 if isprime(4*10**n - 87) else 0 # Indranil Ghosh, Mar 22 2017

Crossrefs

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

Sequence in context: A062879 A272193 A186131 * A065897 A293762 A161889

Adjacent sequences: A284188 A284189 A284190 * A284192 A284193 A284194

Keyword

nonn,more,hard

Author

Robert Price, Mar 22 2017

Status

approved