A278397 Numbers n such that 10^n - 20001 is prime.

5, 11, 16, 21, 37, 83, 94, 299, 318, 467, 622, 707, 1931, 2175, 2189, 2238, 2526, 5202, 10541, 15822, 17407, 19919, 19998, 25407, 96377, 118009

Offset

1,1

Comments

For n>4, numbers such that n-5 occurrences of the digit 9 followed by the digits 79999 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 9w79999.

Example

5 is in this sequence because 10^5 - 20001 = 79999 is prime.
Initial terms and primes associated:
a(1) = 5, 79999;
a(2) = 11, 99999979999;
a(3) = 16, 9999999999979999;
a(4) = 21, 999999999999999979999;
a(5) = 37, 9999999999999999999999999999999979999; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[10^# - 20001] &]

Prog

(Magma) [n: n in [5..400] | IsPrime(10^n - 20001)]; // Vincenzo Librandi, Nov 21 2016
(PARI) is(n)=ispseudoprime(10^n-20001) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

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

Sequence in context: A314137 A314138 A314139 * A314140 A314141 A314142

Adjacent sequences: A278394 A278395 A278396 * A278398 A278399 A278400

Keyword

nonn,more,hard

Author

Robert Price, Nov 20 2016

Extensions

a(26) from Robert Price, Jan 26 2018

Status

approved