%I M3071
%S 1,3,19,271,7365,326011,21295783,1924223799,229714292041,
%T 35007742568755,6630796801779771,1527863209528564063,
%U 420814980652048751629,136526522051229388285611
%N Number of permutations of up to n kinds of objects, where each kind of object can occur at most two times.
%C E.g.f. A(x)=y satisfies 0=(2x^3+2x^2)y''+(3x^3+4x1)y'+(x^3x^22x+3)y.  _Michael Somos_, Mar 15 2004
%C Number of ways to use the elements of {1,..,k}, 0<=k<=2n, once each to form a sequence of n (possibly empty) sets, each having at most 2 elements.  Bob Proctor, Apr 18 2005
%D J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 17.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Robert A. Proctor, <a href="http://arxiv.org/abs/math.CO/0606404">Let's Expand Rota's Twelvefold Way For Counting Partitions!</a>, arXiv:math.CO/0606404, Jan 05, 2007
%H <a href="/index/Par#partN">Index entries for related partitioncounting sequences</a>
%F a(n)n=a(n1)(2n^3n^2+n+1)+a(n2)(3n^3+4n^2+2n3)+a(n3)(n^32n^2n+2).
%F a(n) ~ sqrt(Pi)*2^(n+1)*n^(2*n+1/2)/exp(2*n1).  _Vaclav Kotesovec_, Oct 19 2013
%t Table[nn=2n;a=1+x+x^2/2!;Total[Range[0,nn]!CoefficientList[Series[a^n,{x,0,nn}],x]],{n,0,15}] (* _Geoffrey Critzer_, Dec 23 2011 *)
%o (PARI) a(n)=local(A);if(n<0,0,A=(1+x+x^2/2)^n;sum(k=0,2*n,k!*polcoeff(A,k)))
%Y a(n) = Sum[C(n, k)*A105749(k), 0<=k<=n]
%Y Replace "sequence" by "collection" in comment: A105748.
%Y Replace "sets" by "lists" in comment: A082765.
%K nonn
%O 0,2
%A _N. J. A. Sloane_
%E More terms from _Vladeta Jovovic_, Aug 18 2002
