a(n) = 2*n^n-1.

Terms

a(0) =1a(1) =1a(2) =7a(3) =53a(4) =511a(5) =6249a(6) =93311a(7) =1647085a(8) =33554431a(9) =774840977a(10) =19999999999a(11) =570623341221