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A168444
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Number of partitions of the set {1,2,...,n} such that no block is a sequence of consecutive integers (including 1-element blocks)
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2
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1, 0, 0, 0, 1, 5, 21, 91, 422, 2103, 11226, 63879, 385691, 2461004, 16535820, 116628147, 861033654, 6637143698, 53297137552, 444940442553, 3854539901147, 34592812084693, 321125878230123, 3079144039478532, 30457076370822777, 310407099470429818, 3255972198123974137, 35114803641531204063
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OFFSET
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0,6
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COMMENTS
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Some similar results appear in Klazar (see links).
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REFERENCES
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Richard Stanley, Enumerative Combinatorics, volume 1, second edition, Cambridge Univ Press, 2011, page 192, solution 111.
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LINKS
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FORMULA
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Ordinary g.f.: (1-x)F(x(1-x)), where F(x) = sum_{n>=0} B(n)x^n (the ordinary g.f. for the Bell numbers)
a(n) = b(n)-b(n-1), b(n) = if n=0 then 1 else sum(binomial(k,n-k)*(-1)^(n-k)*B(k),k=ceiling(n/2)..n). - Vladimir Kruchinin, Sep 09 2010
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EXAMPLE
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For n=5 the a(5) = 5 partitions are 13-245, 14-235, 24-135, 25-135, 35-124.
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MAPLE
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with(combinat): y:=sum(bell(n)*x^n, n=0..20): z:=(1-x)*subs(x=x*(1-x), y): taylor(z, x=0, 21);
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MATHEMATICA
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nn = 20; b := Sum[BellB[n] (x - x^2)^n, {n, 0, nn}]; CoefficientList[ Series[ (1-x) b, {x, 0, nn}], x] (* Geoffrey Critzer, Jun 01 2013 *)
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PROG
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(Maxima) b(n):=if n=0 then 1 else sum(binomial(k, n-k)*(-1)^(n-k)*belln(k), k, ceiling(n/2), n); a(n):=if n=0 then 1 else b(n)-b(n-1); /* Vladimir Kruchinin, Sep 09 2010 */
(PARI)
N=66; x = 'x+O('x^N);
B = serlaplace(exp(exp(x)-1));
gf = (1-x)*subst(B, 'x, x*(1-x));
(Magma)
b:= func< n | n eq 0 select 1 else (&+[(-1)^(n+j)*Binomial(j, n-j)*Bell(j): j in [Ceiling(n/2)..n]]) >;
A168444:= func< n | n eq 0 select 1 else b(n)-b(n-1) >;
(SageMath)
@CachedFunction
def b(n): return 1 if (n==0) else sum((-1)^(n+j)*binomial(j, n-j)*bell_number(j) for j in range((n//2), n+1))
def A168444(n): return 1 if (n==0) else b(n) - b(n-1)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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