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!)
A294909 Numbers k such that (10^k + 47)/3 is prime. 0
1, 3, 5, 6, 9, 13, 120, 220, 568, 640, 666, 1205, 1600, 2479, 2482, 2939, 3077, 8649, 9520, 12407, 33421, 36162, 41190, 65340 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k > 1, numbers such that k-2 occurrences of the digit 3 followed by the digits 49 is prime (see Example section). - Robert Price, Nov 10 2017

a(25) > 2*10^5. - Robert Price, Nov 10 2017

LINKS

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

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

Makoto Kamada, Search for 3w49

EXAMPLE

3 is in this sequence because (10^3 + 47)/3 = 349 is prime.

Initial terms and associated primes:

a(1) = 1, 19;

a(2) = 3, 349;

a(3) = 5, 33349;

a(4) = 6, 333349;

a(5) = 9, 333333349; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(10^# + 47)/3] &]

PROG

(MAGMA) [n: n in [0..500] |IsPrime((10^n+47) div 3)];

(PARI) is(n) = ispseudoprime((10^n + 47) / 3); \\ Altug Alkan, Sep 26 2016

CROSSREFS

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

See also similar sequences listed in A274986.

Sequence in context: A096394 A075472 A072522 * A070111 A070117 A176345

Adjacent sequences:  A294906 A294907 A294908 * A294910 A294911 A294912

KEYWORD

nonn,more,hard

AUTHOR

Vincenzo Librandi, Sep 26 2016

EXTENSIONS

a(18)-a(24) from Robert Price, Nov 10 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 September 25 00:01 EDT 2020. Contains 337332 sequences. (Running on oeis4.)