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With the usual euclidean distance, i can't figure out how to solve this... I tried to draw the equality case, and then adding another point at one of the distances already in but of course couldn't find any correct placement

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It is enough to apply the Erdos-Szekeres theorem.

The lines joining two points are finite, so up to rotations we may assume that our points have distinct abscissas.

Assume that we have $M=(n+1)^2+1$ points in the plane $P_1,P_2,\ldots,P_M$, labeled in such a way that $i<j$ implies that the abscissa of $P_i$ is less than the abscissa of $P_j$.

Let $y_1,y_2,\ldots,y_M$ the sequence of the ordinates of $P_1,P_2,\ldots,P_M$. By Erdos-Szekeres this sequence admits a weakly monotonic subsequence $y_{\sigma(1)},y_{\sigma(2)},\ldots,y_{\sigma(n+2)}$, so by considering the distances between $P_{\sigma(1)}$ and $P_{\sigma(2)},\ldots,P_{\sigma(n+2)}$ we have at least $n+1$ different pairwise distances.

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Let $g(N)$ denote the minimal number of distinct distances between $N$ points in the plane. In his 1946 paper, Erdős proved the estimates $$\sqrt{N-3/4}-1/2\leq g(N)\leq c N/\sqrt{\log N}.$$ The lower bound is essentially the question of the OP.

After a long string of improvements, Larry Guth and Nets Katz proved in 2015 the lower bound $$ g(N) \ge cN/ {\log N} $$ that almost matches Erdos' upper bound. See [1] for more details and references.

[1] https://en.wikipedia.org/wiki/Erd%C5%91s_distinct_distances_problem

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