a(n) = (n-3)*2^n + n*(n+3)/2 + 3.

Terms

a(0) =1a(1) =4a(2) =12a(3) =33a(4) =87a(5) =222a(6) =550a(7) =1327a(8) =3129a(9) =7236a(10) =16464a(11) =36957a(12) =82027a(13) =180346a(14) =393354a(15) =852123a(16) =1835181a(17) =3932352a(18) =8388820a(19) =17826025a(20) =37748991a(21) =79692054a(22) =167772462a(23) =352321863a(24) =738197857a(25) =1543504252a(26) =3221225880a(27) =6710886837