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!)
A215659 Values of k such that k*(k - 1) is a primorial number. 5
2, 3, 6, 15, 715 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Values of k in A215658.

See A161620 for the primorial values. - Jeppe Stig Nielsen, Mar 27 2018

LINKS

Table of n, a(n) for n=1..5.

Carol Nelson, David E. Penney and Carl Pomerance, 714 and 715, J. Recreational Math. 7:2 (1994), pp. 87-89.

FORMULA

a(n) * (a(n) - 1) = A215658(n)#, where p# = 2 * 3 * 5 * 7 * 11 * ... * p is a primorial, the product of the primes from 2 to p.

MATHEMATICA

Select[Range[10^5], Product[Prime@ i, {i, PrimeNu@ #}] == # &[# (# - 1)] &] (* Michael De Vlieger, Apr 10 2018 *)

CROSSREFS

Cf. A215658, A161620.

Sequence in context: A014627 A145781 A109162 * A028688 A030753 A003040

Adjacent sequences:  A215656 A215657 A215658 * A215660 A215661 A215662

KEYWORD

nonn,more

AUTHOR

Jonathan Sondow, Sep 07 2012

EXTENSIONS

Name improved by Jeppe Stig Nielsen, Mar 27 2018

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 March 30 19:49 EDT 2020. Contains 333127 sequences. (Running on oeis4.)