market structure	number	nature of	ease of	other
	of firms	the product	entry / exit
monopoly	one	unique	costly,	imperfect
			barriers to entry	information
oligopoly	few,	differentiated	costly	imperfect
	at least some large	or homogeneous		information
monopolistic competition	many, all small	differentiated	easy	imperfect infor.
perfect competition	many, all small	homogeneous	easy	perfect information