一句英语的语法问题

　　一句英语的语法问题。The density distribution of a cylindrical tube could be deduced from the Clausing function [10], but not directly by using the numerical methods proposed by Gómez-Go et al. and Mohan et al.
上句是请润色公司帮忙修改后的句子。我对后一句划线部分有疑问，即：好像没有主谓结构？各位虫友觉得句子有没问题？

[ Last edited by esub on 2013-3-30 at 15:07 ]
===有问必答===
应该是“but not directly be deduced by ”吧