a(1) = 4, and for any n > 1, a(n+1) is the a(n)-th squarefree number.

Terms

a(0) =4a(1) =5a(2) =6a(3) =7a(4) =10a(5) =14a(6) =21a(7) =33a(8) =53a(9) =85a(10) =138a(11) =222a(12) =366a(13) =599a(14) =985a(15) =1613a(16) =2651a(17) =4357a(18) =7169a(19) =11795a(20) =19401a(21) =31913a(22) =52487a(23) =86347a(24) =142021a(25) =233615a(26) =384277a(27) =632091a(28) =1039741a(29) =1710305