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!)
A242884 Least number k such that (k^k-n^n)/(k-n) is prime or 0 if no such number exists. 0
2, 1, 1, 3, 3, 7, 3, 0, 0, 11, 5, 0, 4, 0, 0, 7, 16, 0, 1, 0, 281, 0, 19, 0, 0, 0, 7, 0, 35, 0, 1, 0, 113, 0, 29, 91, 19, 0, 19, 0, 23, 0, 0, 0, 0, 37, 65, 0, 0, 0, 0, 0, 153, 0, 199, 0, 0, 115, 0, 0, 0, 0, 319, 0, 47, 0, 0, 0, 13, 0, 47, 0, 0, 0, 539, 0, 0, 0, 0, 0, 13, 0, 147 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) = 0 is confirmed for k <= 5000. (Which means that the zeros are at present only conjectural. - N. J. A. Sloane, May 26 2014)

If a(i) = j, then a(j) <= i for all i and j.

LINKS

Table of n, a(n) for n=1..83.

EXAMPLE

(2^2-1^1)/(2-1) = 3 is prime. Thus a(1) = 2.

PROG

(PARI) a(n)=for(k=1, 5000, if(k!=n, s=(k^k-n^n)/(k-n); if(floor(s)==s, if(ispseudoprime(s), return(k)))))

n=1; while(n<100, print(a(n)); n+=1)

CROSSREFS

Sequence in context: A276777 A219876 A216655 * A197219 A300508 A120013

Adjacent sequences:  A242881 A242882 A242883 * A242885 A242886 A242887

KEYWORD

nonn,hard,more

AUTHOR

Derek Orr, May 25 2014

EXTENSIONS

We don't normally allow conjectural terms, except in special circumstances. This is one of those exceptions, for if we included only terms that are known for certain, not much of this sequence would remain. - N. J. A. Sloane, May 31 2014

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 July 5 00:01 EDT 2020. Contains 335457 sequences. (Running on oeis4.)