cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-3 of 3 results.

A092480 Primes of the form 40*R_k + 7, where R_k is the repunit (A002275) of length k.

Original entry on oeis.org

7, 47, 4447, 4444444447, 44444444444444444447, 44444444444444444444444447
Offset: 1

Views

Author

Rick L. Shepherd, Apr 03 2004

Keywords

Comments

Primes of the form ((4*10^k - 31)/9) + 6. - Vincenzo Librandi, Dec 13 2011
The next term has 722 digits. - Harvey P. Dale, Jan 19 2020

Links

Crossrefs

Cf. A056682 (corresponding k).

Programs

  • Magma
    [a: n in [1..720] | IsPrime(a) where a is ((4*10^n-31) div 9)+6 ]; // Vincenzo Librandi, Dec 13 2011
  • Mathematica
    Select[Table[(((4*10^n-31)/ 9)+6),{n,1,800}],PrimeQ] (* Vincenzo Librandi, Dec 13 2011 *)
Select[Table[FromDigits[PadLeft[{7},n,4]],{n,30}],PrimeQ] (* Harvey P. Dale, Jan 19 2020 *)

A099413 Numbers k such that 4*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

0, 1, 2, 4, 10, 20, 26, 722, 1310, 3170, 28934, 66284, 67796, 231254, 338476
Offset: 1

Views

Author

Robert G. Wilson v, Oct 14 2004

Keywords

Comments

Also numbers k such that (4*10^k + 23)/9 is prime.
a(14) > 10^5. - Robert Price, Nov 26 2014

Links

Crossrefs

Cf. A002275, A056682.

Programs

  • Mathematica
    Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 3], Print[n]], {n, 5000}]

Formula

a(n) = A056682(n-1) + 1. - Robert Price, Nov 26 2014

Extensions

a(11)-a(13) from Robert Price, Nov 26 2014
a(14)-a(15) from Kamada data by Tyler Busby, May 03 2024

A173771 Primes of form (4*10^n+23)/9.

Original entry on oeis.org

3, 7, 47, 4447, 4444444447, 44444444444444444447, 44444444444444444444444447
Offset: 1

Views

Author

Vincenzo Librandi, Feb 24 2010

Keywords

Comments

a(9) has 1310 digits. - Bruno Berselli, Jul 26 2012

Links

Crossrefs

Cf. A173772
Equals 3 followed by A092480 (primes of the form 40*R_n + 7). - Klaus Brockhaus and R. J. Mathar, Feb 27 2010
Cf. A056682.

Programs

  • Magma
    [a: n in [0..100] | IsPrime(a) where a is (4*10^n+23)div 9 ]; // Vincenzo Librandi, Jul 26 2012
  • Mathematica
    Select[Table[(4*10^n+23)/9,{n,0,100}],PrimeQ] (* Vincenzo Librandi, Jul 26 2012 *)
Showing 1-3 of 3 results.

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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