map ,... can't work except , why? Vista+gVim V7.2

Hi list,

Does anyone know why the following mapping can't work, and how to make
it enable?

" Windows Vista + gVim V7.2
:map <C--> <C-W>w-
:map <C-=> <C-W>w+

':map <C-->/<C-=>' and ':verbose map <C-->/<C--=>', it's OK. i.e.
it's in the map list and hasn't been modified by others map.

------------------------------------------
i tried any <C+Numeric key>, in main keyboard not in keypad, none
could work except <C-6>(interior map by gVim self).
While mapping <A-1>, <A-2>....<A-=>, all of them can work.

Regards,
-William

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