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A270049
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Number of 123 avoiding set partitions of [n].
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1
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1, 1, 3, 7, 21, 61, 199, 659, 2345, 8569, 32971, 130527, 538045, 2279733, 9987727, 44897131, 207693905, 983926001, 4780294291, 23740460215, 120595843941, 625175300653, 3308054119767, 17837452131139, 98006292402553, 548026191197801, 3118110841312091
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) = Sum_{i=0..n/2} binomial(n,2*i)*(2*i)!!.
a(n) - 2*a(n-1) - (n-1)*a(n-2) + (n-2)*a(n-3) = 0.
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MAPLE
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add( binomial(n, 2*i)*doublefactorial(2*i), i=0..n/2) ;
end proc:
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MATHEMATICA
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Table[Sum[Binomial[n, 2 i] (2 i)!!, {i, 0, n}], {n, 0, 26}] (* Michael De Vlieger, Mar 09 2016 *)
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PROG
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(PARI) a(n)=my(b=1, df=1, t); sum(i=1, n\2, t+=2; b*=(n-t+2)*(n-t+1)/(t*(t-1)); df*=t; b*df)+1 \\ Charles R Greathouse IV, Mar 10 2016
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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