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A250283
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Number of permutations p of [n] such that p(i) > p(i+1) iff i == 0 (mod 6).
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4
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1, 1, 1, 1, 1, 1, 1, 6, 27, 83, 209, 461, 923, 10284, 80991, 414961, 1671853, 5699149, 17116009, 278723178, 3135810159, 22493048843, 124606826189, 574688719793, 2301250545971, 49308397822776, 721175428306971, 6650954153090521, 46893517738791361
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OFFSET
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0,8
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LINKS
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EXAMPLE
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a(6) = 1: 123456.
a(7) = 6: 1234576, 1234675, 1235674, 1245673, 1345672, 2345671.
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MAPLE
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b:= proc(u, o, t) option remember; `if`(u+o=0, 1,
`if`(t=0, add(b(u-j, o+j-1, irem(t+1, 6)), j=1..u),
add(b(u+j-1, o-j, irem(t+1, 6)), j=1..o)))
end:
a:= n-> b(n, 0$2):
seq(a(n), n=0..35);
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MATHEMATICA
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nmax = 30; CoefficientList[Series[1 + Sum[(x^(6 - k) * HypergeometricPFQ[{1}, {7/6 - k/6, 4/3 - k/6, 3/2 - k/6, 5/3 - k/6, 11/6 - k/6, 2 - k/6}, -x^6/46656])/(6 - k)!, {k, 0, 5}] / HypergeometricPFQ[{}, {1/6, 1/3, 1/2, 2/3, 5/6}, -x^6/46656], {x, 0, nmax}], x] * Range[0, nmax]! (* Vaclav Kotesovec, Apr 21 2021 *)
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PROG
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(Sage)
@cached_function
def b(u, o, t):
if u ==-o: return 1
if t == 0: return sum(b(u-j, o+j-1, (t+1) % 6) for j in (1..u))
return sum(b(u+j-1, o-j, (t+1) % 6) for j in (1..o))
a = lambda n: b(n, 0, 0)
print([a(n) for n in (0..28)]) # after Maple program
# Alternatively:
@cached_function
def A(m, n):
if n == 0: return 1
s = -1 if m.divides(n) else 1
t = [m*k for k in (0..(n-1)//m)]
return s*add(binomial(n, k)*A(m, k) for k in t)
A250283 = lambda n: (-1)^int(is_odd(n//6))*A(6, n)
print([A250283(n) for n in (0..28)])
# (End)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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