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A073936
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2^n + 1 is squarefree and has exactly 2 prime factors.
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2
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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LINKS
| AMS Books Online, Factorizations of b^n = +-1, b=2,3,5,6,7,10,11,12 Up to High Powers, Third Edition
Arjen Bot, Factors for 2^n-1 and 2^n+1 for 1200 < n < 10000
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FORMULA
| Solutions to A000005[A000051(x)]=4 or A046798[x]=4
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EXAMPLE
| 11 is a member because 1+2^11=2049=3.683.
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MATHEMATICA
| Do[ If[ Length[ Divisors[1 + 2^n]] == 4, Print[n]], {n, 1, 200}]
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CROSSREFS
| Cf. A000005, A000051, A046798. Different from A066263.
Sequence in context: A131503 A047266 A026309 * A184811 A064615 A139205
Adjacent sequences: A073933 A073934 A073935 * A073937 A073938 A073939
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu), Aug 13 2002
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EXTENSIONS
| Edited by Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 19 2002
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