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A052844
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A simple grammar.
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1
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1, 2, 6, 26, 148, 1032, 8464, 79592, 842832, 9914336, 128162464, 1804852128, 27489582784, 450089665664, 7880963503872, 146913179393408, 2904309329449216, 60677563647195648, 1335634021282590208
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| An unspecified number of sign-in sheets are available at a meeting of n people. The attendees sign in on one of the sheets in the order that they arrive at the meeting. But some, none, or all of the attendees forget to sign in. a(n) is the number of ways this can happen.
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LINKS
| INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 812
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FORMULA
| E.g.f.: exp(x*(-2+x)/(-1+x))
Recurrence: {a(0)=1, a(1)=2, a(2)=6, (-2-n^2-3*n)*a(n)+(n^2+5*n+6)*a(n+1)+(-2*n-6)*a(n+2)+a(n+3)}
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MAPLE
| spec := [S, {B=Sequence(Z, 1 <= card), C=Union(Z, B), S=Set(C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
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MATHEMATICA
| CoefficientList[Series[Exp[x/(1 - x)] Exp[x], {x, 0, 20}], x]*
Table[n!, {n, 0, 20}]
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CROSSREFS
| Sequence in context: A132609 A168450 A125224 * A052859 A103937 A159311
Adjacent sequences: A052841 A052842 A052843 * A052845 A052846 A052847
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KEYWORD
| easy,nonn
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AUTHOR
| encyclopedia(AT)pommard.inria.fr, Jan 25 2000
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