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!)
A295399 Numbers k such that (8*10^k - 611)/9 is prime. 0
2, 3, 4, 7, 9, 48, 52, 105, 153, 1783, 1999, 2787, 4276, 13693, 14067, 19143, 20431, 25933, 26206, 29062, 40462, 40791, 58738, 81709 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that k-2 occurrences of the digit 8 followed by the digits 21 is prime (see Example section).

a(25) > 2*10^5.

LINKS

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 8w21

EXAMPLE

2 is in this sequence because (8*10^2 - 611)/9 = 21 is prime.

Initial terms and primes associated:

a(1) = 2, 21;

a(2) = 3, 821;

a(3) = 4, 8821;

a(4) = 7, 8888821;

a(5) = 9, 888888821; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(8*10^# - 611)/9] &]

PROG

(PARI) isok(k) = isprime((8*10^k - 611)/9); \\ Michel Marcus, Nov 22 2017

CROSSREFS

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

Sequence in context: A071114 A078696 A256772 * A274976 A269729 A098115

Adjacent sequences:  A295396 A295397 A295398 * A295400 A295401 A295402

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 21 2017

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 11 18:34 EST 2019. Contains 329925 sequences. (Running on oeis4.)