On the size of finite Sidon sets

A Sidon set is a set of integers containing no nontrivial solutions to the equation $a+b=c+d$. We improve on the lower bound on the diameter of a Sidon set with $k$ elements: if $k$ is sufficiently large and $\mathcal{A}$ is a Sidon set with $k$ elements, then $diam(\mathcal{A})\ge k^2-1.99405k^{3\mathrm{/}2}$. Alternatively, if $n$ is sufficiently large, then the largest subset of $\{1,2,\dots ,n\}$ that is a Sidon set has cardinality at most $n^{1\mathrm{/}2}+0.99703n^{1\mathrm{/}4}$. While these are only slight numerical improvements on Balogh-Füredi-Roy (arXiv:2103:15850v2), we use a method that is logically simpler.