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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A291201 Numbers k such that (13*10^k - 61)/3 is prime. 0
1, 4, 7, 9, 10, 13, 27, 35, 94, 150, 198, 258, 673, 1194, 1492, 2320, 2727, 3767, 6246, 6877, 14481, 34327, 57634, 123137, 190732 (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 13 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 43w13.

EXAMPLE

4 is in this sequence because (13*10^4 - 61)/3 = 43313 is prime.

Initial terms and primes associated:

a(1) = 1, 23;

a(2) = 4, 43313;

a(3) = 7, 43333313;

a(4) = 9, 4333333313;

a(5) = 10, 43333333313; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[(13*10^# - 61)/3] &]

PROG

(MAGMA) [n: n in [1..300] |IsPrime((13*10^n - 61) div 3)]; // Vincenzo Librandi, Aug 21 2017

CROSSREFS

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

Sequence in context: A010442 A055047 A234429 * A310940 A153001 A010454

Adjacent sequences:  A291198 A291199 A291200 * A291202 A291203 A291204

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Aug 20 2017

EXTENSIONS

a(24)-a(25) from Robert Price, Nov 28 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 February 22 02:13 EST 2019. Contains 320381 sequences. (Running on oeis4.)