First differences of A002002.

Terms

a(0) =4a(1) =20a(2) =104a(3) =552a(4) =2972a(5) =16172a(6) =88720a(7) =489872a(8) =2719028a(9) =15157188a(10) =84799992a(11) =475894200a(12) =2677788492a(13) =15102309468a(14) =85347160608a(15) =483183316512