How to properly train your Self-Organized Map?
An answer from Kohonen, inventor of the self-organized map himself:

&quot;The true mathematical form of σ(t) is not crucial, as long as its
value is fairly large in the beginning of the process. Say, on the
order of half of the diameter of the grid, whereafter it is gradually
reduced to a fraction of it in about 1000 steps.&quot;

From: Kohonen, T., 2013. Essentials of the self-organizing map. Neural networks, 37, pp.52-65.