a(n) = 11 + floor((2 + Sum_{j=1..n-1} a(j))/3).

Terms

a(0) =11a(1) =15a(2) =20a(3) =27a(4) =36a(5) =48a(6) =64a(7) =85a(8) =113a(9) =151a(10) =201a(11) =268a(12) =358a(13) =477a(14) =636a(15) =848a(16) =1131a(17) =1508a(18) =2010a(19) =2680a(20) =3574a(21) =4765a(22) =6353a(23) =8471a(24) =11295a(25) =15060a(26) =20080a(27) =26773a(28) =35697a(29) =47596