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Search: id:A107848
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| A107848 |
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Number of subsets of the n-th roots of unity summing to a real number. |
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+0
2
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| 2, 4, 4, 8, 8, 24, 16, 48, 40, 144, 64, 336, 128, 864, 416, 1728, 512, 8304, 1024, 10656, 4032, 31104, 4096, 116256, 9248, 186624, 40000, 374976, 32768, 3537024, 65536, 2239488
(list; graph; listen)
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OFFSET
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1,1
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LINKS
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T. D. Noe, Sums of Roots of Unity Plots
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FORMULA
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For prime n, a(n)=2^((n+1)/2).
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MATHEMATICA
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Needs["DiscreteMath`Combinatorica`"]; Table[Plus@@Table[Count[(KSubsets[Range[n], k]), q_List/; Im[Chop[Plus@@(E^(2.*Pi*I*q/n))]]==0], {k, 0, n}], {n, 20}]
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CROSSREFS
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Cf. A103314 (number of subsets of the n-th roots of unity summing to zero).
Adjacent sequences: A107845 A107846 A107847 this_sequence A107849 A107850 A107851
Sequence in context: A166632 A116596 A048656 this_sequence A029599 A076466 A073117
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KEYWORD
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nonn
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AUTHOR
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T. D. Noe (noe(AT)sspectra.com), May 25 2005
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