a(n) = Sum_{j = 1..n} Sum_{i = 1..n} (i + j)! / (i! * j!).

Terms

a(0) =0a(1) =2a(2) =14a(3) =62a(4) =242a(5) =912a(6) =3418a(7) =12854a(8) =48602a(9) =184736a(10) =705410a(11) =2704132a(12) =10400574a(13) =40116572a(14) =155117490a(15) =601080358a(16) =2333606186a(17) =9075135264a(18) =35345263762a(19) =137846528780a(20) =538257874398