cp's OEIS Frontend

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).

%I A137802 #3 Mar 30 2012 17:27:00
%S A137802 0,3,150,31185,12999420,9622703475,11539805487210,20981809690466625,
%T A137802 54997428661808232600,199760599884519009411075,
%U A137802 973866344327734952575230750,6207575427404936259602204502225
%N 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).
%F A137802 a(n) = A137801(n) / n! = SUM[i+j<=n] (-1)^i * (2n)! * (2n-i-2j)! / (n-i-j)! / i! / j! / 2^(2n-2i-j)
%o A137802 (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) ) ) }
%Y A137802 Cf. A094047, A137801.
%K A137802 nonn
%O A137802 1,2
%A A137802 _Max Alekseyev_, Feb 10 2008