Number of fixed polyominoes with n cells and tree-like structure.

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =55a(5) =174a(6) =570a(7) =1908a(8) =6473a(9) =22202a(10) =76886a(11) =268352a(12) =942651a(13) =3329608a(14) =11817582a(15) =42120340a(16) =150682450a(17) =540832274a(18) =1946892842a(19) =7027047848a(20) =25424079339a(21) =92185846608a(22) =334925007128