We give improved hitting-sets for two special cases of Read-once Oblivious Arithmetic Branching Programs (ROABP). First is the case of an ROABP with known variable order. The best hitting-set known for this case had cost (nw)^{O(\log n)}, where n is the number of variables and w is the width of the ROABP. Even for a constant-width ROABP, nothing better than a quasi-polynomial bound was known. We improve the hitting-set complexity for the known-order case to n^{O(\log w)}. In particular, this gives the first polynomial time hitting-set for constant-width ROABP (known-order). However, our hitting-set works only over those fields whose characteristic is zero or large enough. To construct the hitting-set, we use the concept of the rank of partial derivative matrix. Unlike previous approaches whose basic building block is a monomial map, we use a polynomial map.
The second case we consider is that of commutative ROABP. The best known hitting-set for this case had cost
d^{O(\log w)}(nw)^{O(\log \log w)}, where d is the individual degree. We improve this hitting-set complexity to (ndw)^{O(\log \log w)}. We get this by achieving rank concentration more efficiently.