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Aug 5, 2013 at 14:16 comment added Will Sawin This is correct. The product is a polynomial in the $\alpha_i$ and $\beta_i$, with the coefficient of $\beta_{i_j}$ a degree $j-1$ monomial in the $\alpha_i$. Thus from the polynomial we can determine the sequence $i_j$, so it is injective.
Aug 5, 2013 at 14:11 comment added Derrick Stolee Thanks! I'm pretty sure you are right with the free semigroup, which is how I developed the construction (that is, until I realized I needed inverses as well).
Aug 5, 2013 at 14:10 vote accept Derrick Stolee
Aug 3, 2013 at 0:21 comment added Michael Zieve Incidentally, while the $\phi_i$ cannot generate a free group when $k>1$, one can probably show that they generate a free semigroup. I would guess that the answer to the original question would be "yes" if one required all the $e_j$ to equal $1$.
Aug 2, 2013 at 23:57 history answered Michael Zieve CC BY-SA 3.0