a(n) = floor(n*phi^3), where phi=(1+sqrt(5))/2.

Terms

a(0) =0a(1) =4a(2) =8a(3) =12a(4) =16a(5) =21a(6) =25a(7) =29a(8) =33a(9) =38a(10) =42a(11) =46a(12) =50a(13) =55a(14) =59a(15) =63a(16) =67a(17) =72a(18) =76a(19) =80a(20) =84a(21) =88a(22) =93a(23) =97a(24) =101a(25) =105a(26) =110a(27) =114a(28) =118a(29) =122