Wat is Market concentration, Herfindahl–Hirschman index?

Market concentration is the degree to which a business’s volume or revenue is held by a few firms, measured by concentration ratios (the $k$ largest firms’ combined share, $\mathrm{CR}_k$) or by the Herfindahl–Hirschman index, the sum of the squared market shares in percent,

from near 0 for many small firms to 10 000 for one (Book 1’s Herfindahl index of venues, on a 0 to 10 000 scale).