a(1)=3; a(n)=floor((20+sum(a(1) to a(n-1)))/6).

Terms

a(0) =3a(1) =3a(2) =4a(3) =5a(4) =5a(5) =6a(6) =7a(7) =8a(8) =10a(9) =11a(10) =13a(11) =15a(12) =18a(13) =21a(14) =24a(15) =28a(16) =33a(17) =39a(18) =45a(19) =53a(20) =61a(21) =72a(22) =84a(23) =98a(24) =114a(25) =133a(26) =155a(27) =181a(28) =211a(29) =246