Definition talk:Magma of Sets

Hi LF. I hope you don't mind these questions.

Question 1 (an obvious one)

Is this line:


 * $\phi_i: \mathcal P \left({X}\right)^{J_i} \to P \left({X}\right)$

supposed to be:


 * $\phi_i: \mathcal P \left({X}\right)^{J_i} \to \mathcal P \left({X}\right)$

Question 2

On lines 7 and 8 you present the defining conditions for a magma of sets. On line 9 you give a logically equivalent definition that on the surface seems miles easier to understand. But I've never come across this.

What do you mean by a set being closed under a mapping?

I am trying to understand this concept using finite $\sigma$ algebras. It's very difficult.

Please understand that I'm not making hard work for you. I just seek to see what you see. --Jshflynn (talk) 22:31, 2 October 2012 (UTC)