  In 2007, Grytczuk conjecture that for any sequence $(\ell_i)_{i\ge1}$ of
alphabets of size $3$ there exists a square-free infinite word $w$ such that
for all $i$, the $i$-th letter of $w$ belongs to $\ell_i$. The result of Thue
of 1906 implies that there is an infinite square-free word if all the $\ell_i$
are identical. On the other, hand Grytczuk, Przyby{\l}o and Zhu showed in 2011
that it also holds if the $\ell_i$ are of size $4$ instead of $3$.
  In this article, we first show that if the lists are of size $4$, the number
of square-free words is at least $2.45^n$ (the previous similar bound was
$2^n$). We then show our main result: we can construct such a square-free word
if the lists are subsets of size $3$ of the same alphabet of size $4$. Our
proof also implies that there are at least $1.25^n$ square-free words of length
$n$ for any such list assignment. This proof relies on the existence of a set
of coefficients verified with a computer. We suspect that the full conjecture
could be resolved by this method with a much more powerful computer (but we
might need to wait a few decades for such a computer to be available).
