login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A291178 Numbers k such that (13*10^k - 37)/3 is prime. 0
1, 2, 4, 7, 8, 26, 64, 116, 123, 157, 178, 288, 328, 1730, 2712, 3244, 3865, 7766, 8792, 9512, 14917, 33912, 39058, 57997, 120306, 150675, 171306, 173467, 175965 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 21 is prime (see Example section).

a(30) > 2*10^5.

LINKS

Table of n, a(n) for n=1..29.

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

Makoto Kamada, Search for 43w21.

EXAMPLE

4 is in this sequence because (13*10^4 - 37)/3 = 43321 is prime.

Initial terms and primes associated:

a(1) = 1, 31;

a(2) = 2, 421;

a(3) = 4, 43321;

a(4) = 7, 43333321;

a(5) = 8, 433333321; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(13*10^# - 37)/3] &]

PROG

(PARI) isok(n) = isprime((13*10^n - 37)/3); \\ Altug Alkan, Aug 21 2017

CROSSREFS

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

Sequence in context: A122979 A122980 A012985 * A023145 A094446 A071790

Adjacent sequences:  A291175 A291176 A291177 * A291179 A291180 A291181

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Aug 19 2017

EXTENSIONS

a(25)-a(29) from Robert Price, Jan 01 2019

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 May 10 05:05 EDT 2021. Contains 343748 sequences. (Running on oeis4.)