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    #1
    Hey!!

    We have the set $A=\{a_1, a_2, \ldots \}$, the $a_i$'s might be finitely or infinitely many.
    We have that $\mathbb{Q}(A)=\left \{\frac{f(a_1, \ldots , a_n)}{g(a_1, \ldots , a_n)} : f,g\in \mathbb{Q}[x_1, \ldots , x_n], g\neq 0, a_1, \ldots , a_n\in A, n\in \mathbb{N}\right \}$.


    We have that $\mathbb{Q}(A)=\bigcup_{n=1}^{\infty}P_n$ where $P_n$ is the set of all polynomials of $\mathbb{Q}(A)$ of degree n. $P_n$ can be represented as a $(n+1)$-tuple of the rational coefficients, right?

    It holds that $|P_n|=|\mathbb{Q}^{n+1}|=|\mathbb{Q}|$.

    Then we have the following:

    $$|\mathbb{Q}(A)|=|\bigcup_{n=1}^{\infty}P_n|\leq \sum_{n=1}^{\infty}|P_n|=\sum_{n=1}^{\infty}|\mathbb{Q}|$$

    Is the last sum correct?
    Last edited by mathmari; December 20th, 2016 at 08:20.

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