a(n) = [x^n] Product_{k=1..n} 1/(x^k*(1-x^k)).

Terms

a(0) =1a(1) =1a(2) =3a(3) =12a(4) =47a(5) =192a(6) =811a(7) =3539a(8) =15765a(9) =71362a(10) =327748a(11) =1524081a(12) =7161629a(13) =33958506a(14) =162312471a(15) =781305581a(16) =3784573140a(17) =18435578714a(18) =90261022638a(19) =443956543235