 We show that the assortment optimization problem under this model is NP-hard and present a fully polynomial-time approximation scheme (FPTAS) for this problem .
<coherence>  We show that the assortment optimization problem under this model is NP-hard and present a fully polynomial-time approximation scheme (FPTAS) for this problem .
 We show that the assortment optimization problem under this model is NP-hard , we present fully polynomial-time approximation scheme (FPTAS) for this problem .