Hacker News Reader

And you need some rule for what summing the elements means, where does it come from, I'm curious? If it's just a formal sum then the elements are incomparable between different spaces - and we can't say that one contains the other.

I think I must understand when you say "X + Y" is the sum of elements that you're not using a formal sum {x} + {y} of each element x, and y but you mean sort of like the union of elements in X and Y?

View on HN

No, the idea is that they are all vectors in the same all-encompassing vector space. You can always sum them. What's missing on the formula is that all of these vector spaces are subspaces of the same larger one. That's implicit, otherwise the sum wouldn't make sense. Think that you are playing inside R^n and juggling a few lower-dimensional subspaces of it.

That makes sense, thanks!

You can think of each vector space as being embedded in the direct sum of the two.

Reply on news.ycombinator.com