%I #10 Nov 29 2015 13:44:42
%S 1,1,1,3,17,104,915,8618,104436,1337282,20709209,336013996,6343919118,
%T 124736306407,2780356513594,64249797198125,1651884203936474,
%U 43874277964032394,1278413274487999471,38372024627757454128,1249821733374560346851,41835183404896657899658
%N Number of down-up parking functions of length n.
%H R. Stanley, <a href="http://math.mit.edu/~rstan/transparencies/parking.pdf">Parking Functions</a>, 2011
%F a(2n) = A260694(2n).
%e a(3) = 3: 212, 213, 312.
%e a(4) = 17: 2121, 2131, 2132, 2141, 2142, 2143, 3121, 3131, 3132, 3141, 3142, 3231, 3241, 4121, 4131, 4132, 4231.
%e a(5) = 104: 21212, 21213, 21214, 21215, ..., 52314, 52412, 52413, 53412.
%Y Cf. A000111, A260694.
%K nonn
%O 0,4
%A _Alois P. Heinz_, Nov 29 2015