a(0) = 0; for n>0, a(n) is the index of the last term in A109812 that is <= 2^n - 1.

Terms

a(0) =0a(1) =1a(2) =4a(3) =16a(4) =28a(5) =76a(6) =162a(7) =418a(8) =650a(9) =1892a(10) =3555a(11) =7252a(12) =20059a(13) =39786a(14) =84653a(15) =174566a(16) =343911a(17) =690189a(18) =1688099a(19) =3366971a(20) =7131089a(21) =14446268a(22) =31036955a(23) =62485240a(24) =145444358a(25) =290692248a(26) =624275567