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A321213
a(n) is the number of divisors of n-th Fermat number (A000215).
0
2, 2, 2, 2, 2, 4, 4, 4, 4, 8, 16, 32
OFFSET
0,1
FORMULA
a(n) = A000005(A000215(n)). - Omar E. Pol, Oct 31 2018
a(n) = 2^A046052(n) for squarefree A000215(n). - Amiram Eldar, Oct 31 2018
EXAMPLE
A000215(n) is prime for n=0 to 4, so a(n)= 2 for n=0 to 4.
MATHEMATICA
Table[DivisorSigma[0, 2^2^n + 1], {n, 120}]
PROG
(PARI) a(n) = numdiv(2^2^n+1)
(Magma) [DivisorSigma(0, 2^2^n + 1): n in [1..100]]
(GAP) List(List([0..11], n->2^(2^n)+1), i->Number(DivisorsInt(i))); # Muniru A Asiru, Nov 03 2018
CROSSREFS
Sequence in context: A001306 A260984 A108105 * A063468 A010336 A054537
KEYWORD
nonn,more,hard
AUTHOR
Jinyuan Wang, Oct 31 2018
EXTENSIONS
a(10)-a(11) from Amiram Eldar, Oct 31 2018
STATUS
approved