A137802 - OEIS

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A137802

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).

0, 3, 150, 31185, 12999420, 9622703475, 11539805487210, 20981809690466625, 54997428661808232600, 199760599884519009411075, 973866344327734952575230750, 6207575427404936259602204502225 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,2`
	FORMULA	`a(n) = A137801(n) / n! = SUM[i+j<=n] (-1)^i * (2n)! * (2n-i-2j)! / (n-i-j)! / i! / j! / 2^(2n-2i-j)`
	PROG	`(PARI) { a(n) = sum(i=0, n, (-1)^i * sum(j=0, n-i, (2n)! (2n-i-2j)! / (n-i-j)! / i! / j! / 2^(2n-2i-j) ) ) }`
	CROSSREFS	`Cf. A094047, A137801.` `Sequence in context: A111816 A157555 A157578 * A100203 A189248 A095225` `Adjacent sequences: A137799 A137800 A137801 * A137803 A137804 A137805`
	KEYWORD	`nonn`
	AUTHOR	`Max Alekseyev (maxale(AT)gmail.com), Feb 10 2008`

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Last modified February 12 15:43 EST 2012. Contains 205431 sequences.