I am trying to go to China to do research for a short period (~2 months). It seems the visa type I should apply is "F". I got an invitation letter from the supervisor in China saying that he is inviting me and all the expenses will be paid.
However when I go to apply for the visa they said that I need to provide an "invitation letter of duly authorized unit". But when I contacted my supervisor it seems to get such a letter we (myself and the university in China) should have a license agreement, etc.
Do you think the visa type (F) I am applying is the right one? If it is so, how to get "invitation letter of duly authorized unit"?