Björner-Welker sequence: 2^n*(n^2 + n + 2) - 1.

Terms

a(0) =1a(1) =7a(2) =31a(3) =111a(4) =351a(5) =1023a(6) =2815a(7) =7423a(8) =18943a(9) =47103a(10) =114687a(11) =274431a(12) =647167a(13) =1507327a(14) =3473407a(15) =7929855a(16) =17956863a(17) =40370175a(18) =90177535a(19) =200278015a(20) =442499071a(21) =973078527a(22) =2130706431a(23) =4647288831a(24) =10099884031a(25) =21877489663