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The preceding bunch of comments by 24.5 make me laugh! I'm a professional mathematician and even have a significant result in mathematical physics. I also am often frustrated when these physics articles are badly written and don't define their terms, not even in blue.  I constantly hammer on about it, either complaining, or fixing, depending on my knowledge of it. Yet at the same time, I strongly recommend following those 5 blue links and reading those 10 or 15 articles. You'll range from the dot product to nuclear spaces. It's the only way. 89.217.26.179  07:52, 28 April 2015