a(n) = denominator(Sum_{k=1..n} (2^(k-1) - 1)/k).

Terms

a(0) =1a(1) =2a(2) =2a(3) =4a(4) =4a(5) =12a(6) =12a(7) =24a(8) =8a(9) =40a(10) =40a(11) =120a(12) =120a(13) =840a(14) =840a(15) =1680a(16) =1680a(17) =5040a(18) =5040a(19) =5040a(20) =5040a(21) =55440a(22) =55440a(23) =55440a(24) =11088a(25) =144144a(26) =48048a(27) =48048a(28) =48048a(29) =80080