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A006551
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Maximal Eulerian numbers.
(Formerly M3426)
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6
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1, 1, 4, 11, 66, 302, 2416, 15619, 156190, 1310354, 15724248, 162512286, 2275172004, 27971176092, 447538817472, 6382798925475, 114890380658550, 1865385657780650, 37307713155613000, 679562217794156938, 14950368791471452636
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OFFSET
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1,3
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COMMENTS
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Define A(n,k) as the number of permutations of {1,2,..,n} with k ascents.
A(n,k) = sum_{j=0}^k (-1)^j binomial(n+1,j)(k-j+1)^n.
Then a(n) = A(n, floor(n/2)). The Digital Library of Mathematical Functions calls the A(n,k) Eulerian numbers. With this terminology a(n) are the middle Eulerian numbers and A180056 the central Eulerian numbers. (End)
Number of permutations of {1,2,..,n} with floor(n/2) descents. - Joerg Arndt, Aug 15 2014
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REFERENCES
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L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 243.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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a(n) = sum_{0<=j<=floor(n/2)} (-1)^j binomial(n+1,j) (floor(n/2)-j+1)^n. [Peter Luschny, Aug 08 2010]
a(n+1)/a(n) ~ n. - Ran Pan, Oct 26 2015
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MAPLE
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a := proc(n) local j, k; k := iquo(n, 2);
add((-1)^j*binomial(n+1, j)*(k-j+1)^n, j=0..k) end:
# Computation by recursion:
A006551 := proc(r) local W; W := proc(m) local A, n, k;
A:=[seq(1, n=1..m)]; if m < 2 then RETURN(1) fi;
for n from 2 to m-1 do for k from 2 to m do
A[k] := n*A[k-1]+k*A[k] od od; [A[m-1], A[m]] end:
W((r+2+irem(r, 2))/2)[2-irem(r, 2)] end:
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MATHEMATICA
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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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