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.

A215780 Primes of the form 2*k!! - 1.

Original entry on oeis.org

3, 5, 29, 1889, 20789, 270269, 15811707161249, 443286190953399543749, 433724868863888852244234239999, 163685683629861106170483229851647999999, 10144121732653119949813597183199268946575359999999, 67476497990875549413061344119283906281177486718749
Offset: 1

Views

Author

Michel Lagneau, Aug 23 2012

Keywords

Comments

The corresponding k are in A215779.

Examples

			29 =  2*5!! - 1 = 2*15 - 1 is in the sequence.

Crossrefs

Cf. A006882, A215779.

Programs

  • Mathematica
    a={}; Do[p= 2*n!! -1; If[PrimeQ[p], AppendTo[a, p]], {n, 0,800}]; Print[a];
Select[2 Range[70]!!-1,PrimeQ] (* Harvey P. Dale, Dec 31 2020 *)

Extensions

Offset changed to 1 by Jinyuan Wang, May 15 2021

A271392 Integers k such that 3*k!!! + 1 is prime where k!!! is A007661(k).

Original entry on oeis.org

2, 4, 5, 8, 9, 15, 16, 23, 27, 32, 34, 35, 38, 40, 46, 54, 57, 83, 87, 97, 162, 165, 223, 235, 282, 488, 503, 575, 673, 823, 857, 885, 965, 1112, 1401, 2288, 2569, 2788, 3133, 3539, 4070, 4654, 5020, 5613, 6720, 7773, 11256, 18023, 22196
Offset: 1

Views

Author

Altug Alkan, Apr 06 2016

Keywords

Comments

Corresponding primes are 7, 13, 31, 241, 487, 87481, 174721, 289027201, 21427701121, ...

Examples

			4 is a term because 3*4!!! + 1 = 13 is prime.

Crossrefs

Cf. A007661, A037083, A084438, A217647, A215779, A271396.

Programs

  • PARI
    is(k) = ispseudoprime(3*prod(i=0, (k-2)\3, k-3*i) + 1); \\ Jinyuan Wang, Jun 09 2021

Extensions

a(47) from Jinyuan Wang, Jun 09 2021
a(48)-a(49) from Michael S. Branicky, Aug 10 2024

A271396 Integers k such that 3*k!!! - 1 is prime where k!!! is A007661(k).

Original entry on oeis.org

0, 1, 2, 4, 5, 6, 7, 8, 10, 17, 25, 28, 31, 37, 38, 39, 46, 47, 49, 55, 67, 82, 85, 94, 98, 115, 120, 129, 167, 214, 216, 267, 293, 580, 732, 857, 993, 1012, 1069, 1308, 1430, 2366, 2974, 4017, 4870, 9034, 9061, 9752, 10657, 13847, 25390
Offset: 1

Views

Author

Altug Alkan, Apr 06 2016

Keywords

Comments

Corresponding primes are 2, 2, 5, 11, 29, 53, 83, 239, 839, 628319, 1825823999, 51123071999, ...

Examples

			4 is a term because 3*4!!! - 1 = 11 is prime.

Crossrefs

Cf. A007661, A037083, A084438, A217647, A215779, A271392.

Programs

  • Mathematica
    Select[Range[0,5000],PrimeQ[3Times@@Range[#,1,-3]-1]&] (* The program generates the first 45 terms of the sequence. *) (* Harvey P. Dale, Mar 29 2025 *)
  • PARI
    is(k) = ispseudoprime(3*prod(i=0, (k-2)\3, k-3*i) - 1); \\ Jinyuan Wang, Jun 09 2021

Extensions

a(46)-a(50) from Jinyuan Wang, Jun 09 2021
a(51) from Michael S. Branicky, Aug 09 2024
Showing 1-3 of 3 results.

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

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