@vql , as I read the text of the US-Japan treaty and the technical explanation ( you have provided ) regarding article 4,
(a) A US citizen ( taxed on world income , no matter where the tax home is ), under this article is not to be taxed by the host country ( this being Japan ).
(b) Because US taxes its citizens/ immigrants ( GreenCard ) on citizenship basis , it will still tax you on your world income irrespective of your tax home. Most countries tax a person on residency basis ( world income for residents and domestic income for non-residents ). There are of course others with in between forms based on many other criteria such as ownership of home etc. etc.
My conclusion is that as a researcher in Japan and meeting eligibility outlined in article 20, Japan will not tax your income while the US will. What I need to check code sections ( for the exact language ) on whether (and if you had been able to qualify for Physical Presence test ) you could use Foreign Earned Income Exclusion . My sense of the code says NO, but ....
Does this make sense ?
Is there more I can do for you ?
