login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A281992 Numbers k such that (2*10^k - 143)/3 is prime. 0
2, 3, 4, 9, 12, 19, 48, 54, 90, 97, 104, 174, 349, 385, 1020, 1294, 1737, 2430, 9040, 14173, 15604, 17943, 37447, 149803, 164043 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers such that k-2 occurrences of the digit 6 followed by the digits 19 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 6w19.

EXAMPLE

3 is in this sequence because (2*10^3 - 143)/3 = 619 is prime.

Initial terms and primes associated:

a(1) = 2, 19;

a(2) = 3, 619;

a(3) = 4, 6619;

a(4) = 9, 666666619;

a(5) = 12, 666666666619; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(2*10^# - 143)/3] &]

CROSSREFS

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

Sequence in context: A145772 A027866 A283596 * A100902 A283447 A034793

Adjacent sequences:  A281989 A281990 A281991 * A281993 A281994 A281995

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Feb 04 2017

EXTENSIONS

a(24)-a(25) from Robert Price, Mar 14 2018

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 7 00:16 EST 2019. Contains 329812 sequences. (Running on oeis4.)