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A322281
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Number of permutations sigma such that |sigma(i+j)-sigma(i)| >= 4 for 1 <= i <= n - j, 1 <= j <= 3.
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3
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1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 74, 2424, 93424, 4394386, 201355480
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OFFSET
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0,17
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COMMENTS
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2 | a(n) for n > 1.
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LINKS
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EXAMPLE
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a(16) = 2: [4,8,12,16,3,7,11,15,2,6,10,14,1,5,9,13] and its reverse.
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PROG
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(Ruby)
def check(d, a, i)
return true if i == 0
j = 1
d_max = [i, d - 1].min
while (a[i] - a[i - j]).abs >= d && j < d_max
j += 1
end
(a[i] - a[i - j]).abs >= d
end
def solve(d, len, a = [])
b = []
if a.size == len
b << a
else
(1..len).each{|m|
s = a.size
if s == 0 || (s > 0 && !a.include?(m))
if check(d, a + [m], s)
b += solve(d, len, a + [m])
end
end
}
end
b
end
(0..n).map{|i| solve(4, i).size}
end
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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