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Search: id:A068018
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| A068018 |
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Number of fixed points in all 132- and 213-avoiding permutations of {1,2,...,n} (these are permutations with runs consisting of consecutive integers). |
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+0
1
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| 1, 2, 4, 6, 12, 18, 40, 62, 148, 234, 576, 918, 2284, 3650, 9112, 14574, 36420, 58266, 145648, 233030, 582556, 932082, 2330184, 3728286, 9320692, 14913098, 37282720, 59652342, 149130828, 238609314, 596523256, 954437198, 2386092964
(list; graph; listen)
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OFFSET
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1,2
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FORMULA
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a(n)=2^n/4-(-2)^n/36+2*n/3-2/9; ogf=z(1-3z^2)/[(1-4z^2)(1-z)^2]
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EXAMPLE
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a(3)=4 because the permutations 123,231,312,321 of {1,2,3} contain 4 fixed points altogether (all three entries of the first permutations and the entry 2 in the last one.
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MAPLE
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seq(2^n/4-(-2)^n/36+2*n/3-2/9, n=1..40);
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CROSSREFS
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Adjacent sequences: A068015 A068016 A068017 this_sequence A068019 A068020 A068021
Sequence in context: A133488 A068911 A094769 this_sequence A060798 A134320 A107383
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Mar 22 2002
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