Hafnian of a +/-1 array.

Terms

a(0) =1a(1) =-1a(2) =-1a(3) =5a(4) =17a(5) =-121a(6) =-721a(7) =6845a(8) =58337a(9) =-698161a(10) =-7734241a(11) =111973685a(12) =1526099057a(13) =-25947503401a(14) =-419784870961