Problems & Puzzles:
Puzzles
Puzzle 181. (N+k)/k
primes
I received from Phil Carmody the
following nice puzzle:
Find the least N such
that (N+k)/k is prime for k=1, 2, ..., n, for a given value n
The already known results are:
n |
N |
Author |
1 |
1 |
Jack Brennen |
2 |
2 |
" " |
3 |
12 |
" " |
4 |
12720 |
" " |
5 |
19440 |
" " |
6 |
5516280 |
" " |
7 |
5516280 |
" " |
8 |
7321991040 |
" " |
9 |
363500177040 |
" " |
10 |
2394196081200 |
" " |
11 |
3163427380990800 |
" " |
12 |
22755817971366480 |
Phil Carmody |
13 |
3788978012188649280 |
" " |
14 |
2918756139031688155200 |
J. K. Andersen (20/4/03) |
Example:
for n=3, (12+1)/1, (12+2)/2 & (12+3)/3
are primes while (12+4)/4 is composite
Phil warns me that:
"Jack
Brennen found terms up to a=11, and I found the a=12 and a=13
term, but Jud claims he recognizes it from somewhere else - have
you
seen it before? My length 13 result was only 30 minutes work, so
maybe if it's not an old puzzle, it could be turned into a new one!"
Question:
1) Do you know if this sequence has been studied and
published before and where?
2) Can you find the next 3 members of the sequence?
Solution:
J. K. Andersen found (20/4/03)
the solution for n=14:
I agree with the given results and
add:
The smallest solution for n=14 is N=2918756139031688155200 (22 digits).
Found in 8 days with the C program written for puzzle 206.
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