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Search: id:A132345
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| A132345 |
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Number of increasing three-term geometric sequences from the integers {1,2,...,n}. |
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+0
1
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| 0, 0, 0, 1, 1, 1, 1, 2, 4, 4, 4, 5, 5, 5, 5, 8, 8, 10, 10, 11, 11, 11, 11, 12, 16, 16, 18, 19, 19, 19, 19, 22, 22, 22, 22, 27, 27, 27, 27, 28, 28, 28, 28, 29, 31, 31, 31, 34, 40, 44, 44, 45, 45, 47, 47, 48, 48, 48, 48, 49, 49, 49, 51, 58, 58, 58, 58, 59, 59, 59, 59, 64, 64, 64, 68
(list; graph; listen)
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OFFSET
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1,8
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FORMULA
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a(n)=sum_{1<p^2<=n} phi(p)trunc(n/p^2) where phi is Euler's phi function and trunc is the greatest integer function.
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EXAMPLE
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a(24)=12 as the sequences counted are 1,2,4; 2,4,8; 3,6,12; 4,8,16; 5,10,20; 6,12,24; 1,3,9; 2,6,18; 4,6,9; 8,12,18; 1,4,16; 9,12,16
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MAPLE
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sum(numtheory[phi](p)*trunc(n/p^2), p=2..trunc(sqrt(n)));
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CROSSREFS
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Sequence in context: A097154 A108421 A104058 this_sequence A130766 A035633 A084291
Adjacent sequences: A132342 A132343 A132344 this_sequence A132346 A132347 A132348
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KEYWORD
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easy,nonn
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AUTHOR
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David Angell (angell(AT)maths.unsw.edu.au), Nov 07 2007
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