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A137802
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Number of arrangements of 2n couples into n cars such that each car contains 2 men and 2 women but no couple (cars are unlabeled).
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1
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0, 3, 150, 31185, 12999420, 9622703475, 11539805487210, 20981809690466625, 54997428661808232600, 199760599884519009411075, 973866344327734952575230750, 6207575427404936259602204502225
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OFFSET
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1,2
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LINKS
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FORMULA
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a(n) = A137801(n) / n! = SUM[i+j<=n] (-1)^i * (2n)! * (2n-i-2j)! / (n-i-j)! / i! / j! / 2^(2n-2i-j)
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PROG
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(PARI) { a(n) = sum(i=0, n, (-1)^i * sum(j=0, n-i, (2*n)! * (2*n-i-2*j)! / (n-i-j)! / i! / j! / 2^(2*n-2*i-j) ) ) }
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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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