is it possible

[drmurat]

is it possible ( 2 ^ n ) +1 and ( 2 ) ^ ( n + 1 ) + 1 can be prime

[rogue]

Originally Posted by drmurat

is it possible ( 2 ^ n ) +1 and ( 2 ) ^ ( n + 1 ) + 1 can be prime

Yes, for n = 1.

[drmurat]

Originally Posted by rogue

Yes, for n = 1.

lol
and one more sample ?

[paulunderwood]

Originally Posted by drmurat

lol
and one more sample ?

n=0.

[drmurat]

Originally Posted by paulunderwood

n=0.

yes but lets try n> 1

[paulunderwood]

For n > 1, N = 2^n +1 has to be a generalized Fermat prime with b=2 i.e 2^(2^a)+1, but 2^a+1 can never be a power of 2

[sweety439]

Originally Posted by drmurat

is it possible ( 2 ^ n ) +1 and ( 2 ) ^ ( n + 1 ) + 1 can be prime

k*2^n+1 and k*2^(n+1)+1 can be both prime only for k divisible by 3, or one of these two numbers will be divisible by 3.

[drmurat]

Originally Posted by sweety439

k*2^n+1 and k*2^(n+1)+1 can be both prime only for k divisible by 3, or one of these two numbers will be divisible by 3.

can you give sample

[drmurat]

yes it is impossible . one of rhem is diveded by 3 all the time

[carpetpool]

Cunningham Chain of the second kind

Originally Posted by PrimePages

A Cunningham chain of length k (of the first kind) is sequence of k primes, each which is twice the preceding one plus one. For example, {2, 5, 11, 23, 47} and {89, 179, 359, 719, 1439, 2879}.

A Cunningham chain of length k (of the second kind) is a sequence of k primes, each which is twice the preceding one minus one. (For example, {2, 3, 5} and {1531, 3061, 6121, 12241, 24481}.)
.

OP is mentioning a special case of this (k=2, Fermat Primes), there are only finitely such primes as Paul mentioned.

[drmurat]

Originally Posted by carpetpool

Cunningham Chain of the second kind



OP is mentioning a special case of this (k=2, Fermat Primes), there are only finitely such primes as Paul mentioned.

I asked it because if 2^(n) + 1 is prime and 2^(n+1) + 1 can be prime . it means
(2^n) * (2^ (2n) + 1) *(2^ (2n+1) + 1)
gives perfect number
but it is impossible . one of them is devided by 3