A Jacobsthal variant.

Terms

a(0) =0a(1) =1a(2) =1a(3) =5a(4) =7a(5) =21a(6) =35a(7) =85a(8) =155a(9) =341a(10) =651a(11) =1365a(12) =2667a(13) =5461a(14) =10795a(15) =21845a(16) =43435a(17) =87381a(18) =174251a(19) =349525a(20) =698027a(21) =1398101a(22) =2794155a(23) =5592405a(24) =11180715a(25) =22369621a(26) =44731051a(27) =89478485a(28) =178940587a(29) =357913941