World Travel City Photography

(3) O = I2 / 16. This equation (solving for s in (1) and substituting in (2)) shows that with twice the inputs, you produce quadruple the output.
Also, economies of scale:
(4) I = 4O1 / 2. This equation (solving for I in equation (3)) shows that the same output requires less input.
"Cities, then, economize on protection, and so protection against marauding barbarian armies is one reason why people have come together to live in cities..." (O'Flaherty 2005, p. 13).