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Suppose $G$ and $H$ are two divisible abelian groups. Futhermore we have $G\oplus G$=$H\oplus H$. Is $G$ isomorphic to $H$?

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    Related: http://math.stackexchange.com/questions/277442016-08-22

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If $A$ is a divisible abelian group, and we let $\delta_{\infty}(A) = \dim_{\mathbb{Q}}(A/A_{\mathrm tor})$, and $\delta_p=\dim_{\mathbf{F}_p}(A[p])$, where $p$ is a prime and $A[p] = \{a\in A\mid pa = 0\}$, then two divisible groups $A$ and $B$ are isomorphic if and only if $\delta_{\infty}(A)=\delta_{\infty}(B)$ and $\delta_p(A)=\delta_p(B)$ for all primes $p$. Apply it to $G$ and $H$, using the fact that $G\oplus G \cong H\oplus H$.

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    @Basil: Yes: either both $a$ and $b$ are finite, in which case you have $2a=2b$, hence $a=b$; or else both are infinite, in which case you have $a = a+a = b+b= b$ (since for infinite cardinals, $\kappa+\kappa=\kappa$).2011-05-19