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A073936
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Numbers k such that 2^k + 1 is the product of two distinct primes.
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6
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5, 6, 7, 11, 12, 13, 17, 19, 20, 23, 28, 31, 32, 40, 43, 61, 64, 79, 92, 101, 104, 127, 128, 148, 167, 191, 199, 256, 313, 347, 356, 596, 692, 701, 1004, 1228, 1268, 1709, 2617, 3539, 3824, 5807, 10501, 10691, 11279, 12391, 14479, 42737, 83339, 95369, 117239
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OFFSET
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1,1
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COMMENTS
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Original name: "2^n + 1 is squarefree and has exactly 2 prime factors."
As 3 divides 2^a(n) + 1 for any odd term a(n), all odd terms are prime and exactly the Wagstaff primes (A000978), at the exclusion of 3 (which gives 2^3 + 1 = 3^2 not squarefree).
For the even terms, let a(n) = d * 2^j with d odd integer and j > 0. If d > 1, as (2^2^j)^q + 1 divides 2^a(n) + 1 for any odd prime q dividing d, then d must be prime.
So the even terms are all given by the following two class:
a) (d = 1) a(n) = 2^j such that Fj is a semiprime Fermat number. Up to now, only j = 5, 6, 7, 8 are known to give a Fermat semiprime, giving the even terms 32, 64, 128 and 256. We are also assured that 2^j is not a term for j = 9..19 because Fj is not a semiprime for those value of j (see Wagstaf's link). F20 is the first composite Fermat number which could give another even term (it would be 2^20 = 1048576). However, it seems highly unlikely that other Fermat semiprimes could exist.
b) (d = p odd prime) a(n) = p * 2^j with j such that Fj is a Fermat prime and p a prime verifying ((Fj - 1)^p + 1)/Fj is a prime.
Exemplifying that, we have:
for j = 1 this gives only the even term a(2) = 2 * 3 = 6 (see Jack Brennen's result in ref),
for j = 2 we have all the terms of type 2^2 * A057182.
for j = 3 the even terms are of type 2^3 * A127317.
For j = 4 at least up to 200000, there is only the term a(41) = 2^4 * 239 = 3824 (see comment in A127317).
All terms after a(50) refer to probabilistic primality tests for 2^a(n) + 1 (see Caldwell's link for the list of the largest certified Wagstaff primes).
After a(56), from the above, the primes 267017, 269987, 374321, 986191, 4031399 and the even value 4101572 are also terms, but still remains the (remote) possibility of some gaps in between. In addition, 13347311 and 13372531 are also terms, but possibly much further in the numbering (see comments in A000978).
(End)
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LINKS
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AMS Books Online, Factorizations of b^n = +-1, b=2,3,5,6,7,10,11,12 Up to High Powers, Third Edition.
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FORMULA
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EXAMPLE
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11 is a member because 1 + 2^11 = 2049 = 3 * 683.
9 is not a term because 1 + 2^9 = 513 = 3^3 * 19
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MATHEMATICA
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Do[ If[ Length[ Divisors[1 + 2^n]] == 4, Print[n]], {n, 1, 200}]
(* Second program: *)
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PROG
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(Sage) [n for n in xsrange(3, 200) if sigma(2^n+1, 0)==4]
# Second program (faster):
(Sage) v=[]; N=2000
for n in xsrange(4, N):
j=valuation(n, 2)
if j<5:
Fj=2^2^j+1; p=ZZ(n/2^j); q=ZZ((2^n+1)/Fj)
if p.is_prime() and q.is_prime(proof=false): v.append(n)
elif j<9 and n.is_power_of(2): v.append(n)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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