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A293842 Numbers k such that 3*10^k - 31 is prime. 0
2, 3, 5, 10, 21, 101, 133, 187, 209, 363, 747, 1550, 3715, 3875, 6022, 9729, 10460, 12316, 14421, 19023, 50727 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 69 is prime (see Example section).

a(22) > 2*10^5.

LINKS

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 29w69

EXAMPLE

3 is in this sequence because 3*10^3 - 31 = 2969 is prime.

Initial terms and primes associated:

a(1) = 2, 269;

a(2) = 3, 2969;

a(3) = 5, 299969;

a(4) = 10, 29999999969;

a(5) = 21, 2999999999999999999969; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[3*10^# - 31] &]

CROSSREFS

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

Sequence in context: A022861 A001646 A103595 * A014844 A307264 A329244

Adjacent sequences:  A293839 A293840 A293841 * A293843 A293844 A293845

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Oct 18 2017

STATUS

approved

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Last modified June 1 02:09 EDT 2020. Contains 334758 sequences. (Running on oeis4.)