There is a large literature, too long to review here. Deheuvels 1989 and Chistyakov 1996 come to mind. But also try: https://www.google.com/search?btnG=1&pws=0&q=asymptotic+expansion+for+sum+of+independent+and+nonidentically+distributed+%20random+variables#pws=0&q=asymptotic+expansion+for+sum+of+independent+and+non+identically+distributed+random+variables&spell=1 and consider the relevant hits. Hope this helps.

There is a large literature, too long to review here. Deheuvels 1989 and Chistyakov 1996 come to mind. But also try: Google search and consider the relevant hits. Hope this helps.

There is a large literature, too long to review here. Deheuvels 1989 and Chistyakov 1996 come to mind. But also try: http://bit.ly/19t4HGa and consider the relevant hits. Hope this helps.

There is a large literature, too long to review here. Deheuvels 1989 and Chistyakov 1996 come to mind. But also try: https://www.google.com/search?btnG=1&pws=0&q=asymptotic+expansion+for+sum+of+independent+and+nonidentically+distributed+%20random+variables#pws=0&q=asymptotic+expansion+for+sum+of+independent+and+non+identically+distributed+random+variables&spell=1 and consider the relevant hits. Hope this helps.