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!)
A097970 Numbers n such that 7*10^n + 3 is prime. 2
1, 4, 6, 16, 22, 39, 103, 163, 240, 1048, 1974, 2559, 5880, 14436, 28338, 32796, 38079, 56779, 91215, 111325, 115863, 138604 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(23) > 2*10^5. - Robert Price, Aug 15 2015

LINKS

Table of n, a(n) for n=1..22.

Makoto Kamada, Prime numbers of the form 700...003.

Sabin Tabirca and Kieran Reynolds, Lacunary Prime Numbers.

Index entries for primes involving repunits.

FORMULA

a(n) = A101129(n) + 1.

EXAMPLE

For n = 1 we have 7*10^1+3 =7*10+3 = 73, which is prime.

MATHEMATICA

Do[ If[ PrimeQ[ 7*10^n + 3], Print[ n ]], {n, 0, 10000}] (* Robert G. Wilson v, Jan 19 2005 *)

Flatten[Position[Table[7 10^n+3, {n, 100000}], _?PrimeQ]] (* Harvey P. Dale, Nov 01 2011 *)

Select[Range[15000], PrimeQ[7 10^# + 3] &] (* Vincenzo Librandi, Aug 16 2015 *)

PROG

(PARI) main(m)=select(n->isprime(7*10^n + 3), vector(m, i, i)) \\ Anders Hellström, Aug 15 2015

(MAGMA) [n: n in [0..500] | IsPrime(7*10^n + 3)]; // Vincenzo Librandi, Aug 16 2015

CROSSREFS

Cf. A101129.

Sequence in context: A120542 A132344 A306905 * A044858 A323043 A120375

Adjacent sequences:  A097967 A097968 A097969 * A097971 A097972 A097973

KEYWORD

more,nonn

AUTHOR

Julien Peter Benney (jpbenney(AT)ftml.net), Sep 07 2004

EXTENSIONS

a(10)-a(13) from Robert G. Wilson v, Jan 19 2005

a(14)-a(19) from Kamada data by Robert Price, Dec 14 2010

a(20)-a(22) from Robert Price, Aug 15 2015

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 12 20:12 EST 2019. Contains 329961 sequences. (Running on oeis4.)