a(n) = (2^n/n!)*Product_{k=0..n-1} (4*k - 1).

Terms

a(0) =1a(1) =-2a(2) =-6a(3) =-28a(4) =-154a(5) =-924a(6) =-5852a(7) =-38456a(8) =-259578a(9) =-1788204a(10) =-12517428a(11) =-88759944a(12) =-636112932a(13) =-4599585816a(14) =-33511268088a(15) =-245749299312