(Seven has been experimentally determined, but I can't recall where.)
I can't quote the formula here, but HHI is, apparently, the sum of squared percentage market shares. Six equal-sized suppliers give an HHI of approximately 1,667, while seven give approximately 1,429—on opposite sides of the 2010 threshold.
Is that where you got the number?
> In assembling information to be collectively provided to purchasers, providers need to be aware of the potential antitrust consequences of information exchanges among competitors.
> the collection is managed by a third party (e.g., a purchaser, government agency, health care consultant, academic institution, or trade association);
> although current fee-related information may be provided to purchasers, any information that is shared among or is available to the competing providers furnishing the data must be more than three months old; and
> for any information that is available to the providers furnishing data, there are at least five providers reporting data upon which each disseminated statistic is based, no individual provider's data may represent more than 25 percent on a weighted basis of that statistic, and any information disseminated must be sufficiently aggregated such that it would not allow recipients to identify the prices charged by any individual provider.
> The conditions that must be met for an information exchange among providers to fall within the antitrust safety zone are intended to ensure that an exchange of price or cost data is not used by competing providers for discussion or coordination of provider prices or costs.
Damn. From 1996!
I'm not sure I understand where you are going with this? Are you suggesting that a new product cannot enter a market without a number of simultaneous competitors?
If I want to roll out fibre to an area that doesn't have it, can I? If I want to build a fresh-produce store in a town that doesn't have one, then I need to find someone else to compete with?
Lots and lots of businesses operate as quasi-monopolies if you look hard enough. Especially in smaller and rural environments. Lots and lots of places have one gas station, one mechanic, one vet, one doctor, one diner and so on.
Your use of the word "require" seems difficult to implement. What if no-one wants to compete with me? If Apple stops making Macs, does MS have to stop making Windows?