require vs dofile (lua 5.1.4 windows)

["mos" <mmosquito@...>]

Hi!
    there are 2 files in 2 fold like:
a 
  a.lua
b 
  b.lua

the b.lua use
  require "..\\a\\a.lua"
will get a error can not find a.lua

but use
  dofile("..\\a\\a.lua")
will ok

I don't konw why.

Best Regard
yujiang