login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A280449 Numbers n such that (2*10^n - 71)/3 is prime. 0
2, 3, 5, 6, 8, 11, 15, 25, 38, 54, 101, 105, 158, 295, 303, 389, 482, 558, 693, 1965, 3503, 9818, 13255, 30351, 51285, 54767, 57384, 88808, 109645, 174301, 187062 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For n>1, numbers such that n-2 occurrences of the digit 6 followed by the digits 43 is prime (see Example section).

a(32) > 2*10^5.

LINKS

Table of n, a(n) for n=1..31.

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

Makoto Kamada, Search for 6w43.

EXAMPLE

5 is in this sequence because (2*10^5 - 71) / 3 = 66643 is prime.

Initial terms and primes associated:

a(1) = 2, 43;

a(2) = 3, 643;

a(3) = 5, 66643;

a(4) = 6, 666643;

a(5) = 8, 66666643; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(2*10^# - 71) / 3] &]

PROG

(PARI) is(n)=isprime((2*10^n - 71)/3) \\ Charles R Greathouse IV, Jan 03 2017

CROSSREFS

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

Sequence in context: A117517 A339732 A098491 * A107947 A120768 A271109

Adjacent sequences: A280446 A280447 A280448 * A280450 A280451 A280452

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 03 2017

EXTENSIONS

a(29) - a(31) from Robert Price, Dec 31 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified January 26 16:14 EST 2023. Contains 359833 sequences. (Running on oeis4.)