Number at middle of segment n of A078783.

Terms

a(0) =3a(1) =7a(2) =14a(3) =26a(4) =49a(5) =92a(6) =177a(7) =346a(8) =683a(9) =1356a(10) =2699a(11) =5384a(12) =10753a(13) =21490a(14) =42963a(15) =85908a(16) =171797a(17) =343574a(18) =687127a(19) =1374232a(20) =2748439a(21) =5496852a(22) =10993677a(23) =21987326a(24) =43974623a(25) =87949216a(26) =175898401a(27) =351796770a(28) =703593507a(29) =1407186980