Number of n-ominoes plus number of n-iamonds.

Terms

a(0) =2a(1) =2a(2) =2a(3) =3a(4) =8a(5) =16a(6) =47a(7) =132a(8) =435a(9) =1445a(10) =5103a(11) =18259a(12) =66934a(13) =247826a(14) =928137a(15) =3500559a(16) =13290552a(17) =50712016a(18) =194358380a(19) =747624825a(20) =2885120934a(21) =11164896416a(22) =43313276948a(23) =168400053019a(24) =656028153120