Hilbert-Warren Add Sequence.

Terms

a(0) =2a(1) =5a(2) =20a(3) =51a(4) =312a(5) =740a(6) =15000a(7) =34125a(8) =160720a(9) =4988592a(10) =180774720a(11) =844074000a(12) =27991656000a(13) =66264953040