1. 如果解释为国内居民的Investment总额，即I=Id+If，也就是国内居民购买的国内Investment+国内居民购买的国际Investment，那么请问，NCO指的是买了什么？

2.如果解释为国内居民购买的国内Investment的总额，即Id，那就有悖于前面的理论了。

那么我想问的是，如果I已经把国内居民购买的Investment（建厂组织生产，购买国外的股票、债券和基金等等）算进去了，那请问流出去的那一部分NCO，是到哪里去了呢？


———————— 赫赫， 你就盯上 Investment 了，别忘了还有 C  G ，它们都是你不理解的NCO中的因素。

First of all, if i remember correctly, Mankiw clarifies and emphasizes more than once in textbook the investment here means investment on physical capital , or on physical product factors.
Secondly, you quoted: "When a nation’s saving exceeds its domestic investment,..." from Mankiw, which implies it's fine to count investment on foreign physical capitals as I, but the necessary condition is u have to bring those physical product factors into your country , otherwise, u aren't importing anything physical in, thus isn't benefit domestic economy or gross domestic product.
Now check on some examples: assume u are rich, successful and interested enough, u buy machines from US, Germany and Japan, and bring those into China, then those investment counts toward I. On the other hand, if u trade S&P 500 and NASDAQ in NYSE, invest on US Treasury bills or bonds, purchase real estates in Europe, or buy natural gas deposits in Russia and gold and diamond mines in South Africa, which, in one way or another, all count as investments, but by no means u could bring physical capitals into your own country from, then those should be counted as capital outflow rather than the investment I, because in the end, in exchanging the cash paid out, u claim ownership on foreign physical capitals, which only benefits foreign economies and count as investments I in foreign countries, no matter who actually owns those assets.

你的意思是说，I只包括本国居民购买在国内的domestic factors of production和进口到国内的foreign factors of production？而通过兑换外币去购买在国外的foreign factors of production则要归入Capital Outflow中吗？

那同理，C和G只包括本国居民购买在国内的domestic goods & services和进口到国内的foreign goods & services？

按照你这么说去理解整个流程的话，确实一切都说得通了。。。但我理解的那个意思就是你说的那个意思吗？

我修改了帖子正文的表述，请你看看我说的有错吗？