

A322281


Number of permutations sigma such that sigma(i+j)sigma(i) >= 4 for 1 <= i <= n  j, 1 <= j <= 3.


3



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

0,17


COMMENTS

2  a(n) for n > 1.


LINKS

Table of n, a(n) for n=0..21.


EXAMPLE

a(16) = 2: [4,8,12,16,3,7,11,15,2,6,10,14,1,5,9,13] and its reverse.


PROG

(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
def A322281(n)
(0..n).map{i solve(4, i).size}
end
p A322281(18)


CROSSREFS

Cf. A179957, A279214.
Sequence in context: A083018 A240174 A295172 * A282878 A062552 A116172
Adjacent sequences: A322278 A322279 A322280 * A322282 A322283 A322284


KEYWORD

nonn,more


AUTHOR

Seiichi Manyama, Dec 01 2018


EXTENSIONS

a(21) from Alois P. Heinz, Dec 02 2018


STATUS

approved



