Number of length-n binary words having borders with two mismatches.

Terms

a(0) =0a(1) =0a(2) =2a(3) =8a(4) =22a(5) =46a(6) =98a(7) =210a(8) =430a(9) =886a(10) =1790a(11) =3638a(12) =7350a(13) =14830a(14) =29758a(15) =59802a(16) =119802a(17) =240362a(18) =480966a(19) =963302a(20) =1927382a(21) =3857746a(22) =7715446a(23) =15437078a(24) =30873042a(25) =61759618a(26) =123512490a(27) =247051278a(28) =494077866a(29) =988213906