a(1)=0, a(n+1) is smallest nonprime > 2*a(n).

Terms

a(0) =0a(1) =1a(2) =4a(3) =9a(4) =20a(5) =42a(6) =85a(7) =171a(8) =343a(9) =687a(10) =1375a(11) =2751a(12) =5504a(13) =11009a(14) =22019a(15) =44039a(16) =88080a(17) =176162a(18) =352325a(19) =704651a(20) =1409303a(21) =2818607a(22) =5637215a(23) =11274431a(24) =22548863a(25) =45097727a(26) =90195455a(27) =180390911a(28) =360781823a(29) =721563647