|
|
A238694
|
|
Smallest k such that 2^n - k and k*2^n - 1 are both prime or 0 if no such k exists.
|
|
5
|
|
|
0, 1, 1, 3, 1, 3, 1, 5, 25, 5, 31, 5, 1, 15, 49, 17, 1, 5, 1, 17, 9, 33, 69, 89, 61, 111, 199, 309, 75, 297, 1, 5, 49, 131, 31, 17, 31, 131, 165, 437, 55, 33, 309, 495, 361, 437, 999, 89, 139, 195, 129, 183, 685, 315, 915, 189, 585, 1035, 931, 93, 1, 57, 165
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,4
|
|
COMMENTS
|
If a(n)=1, then the two primes are same and they are Mersenne primes (A000668).
|
|
LINKS
|
|
|
EXAMPLE
|
a(9) = 25 because 2^9 - 25 = 487 and 25*2^9 - 1 = 12799 are both prime.
|
|
MAPLE
|
a:= proc(n) local k, p; p:= 2^n;
for k while not (isprime(p-k) and isprime(k*p-1))
do if k>=p then return 0 fi od; k
end:
|
|
MATHEMATICA
|
a[n_] := Module[{k, p}, p = 2^n;
For[k = 1, !(PrimeQ[p - k] && PrimeQ[k*p - 1]), k++,
If[k >= p, Return[0]]]; k];
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|