Sasha Frere-Jones has an equation for how music gets sold in today's market:

Song X + Apple commercial + Google (no credits in ads? no problem) = No. 1 song on iTunes.

Though I do have to bitch that the New Yorker refers to this as a calculus of music when it is clearly a very simple algebraic equation. Now the word calculus can refer simply to a calculation, but this use of the word annoys me. Unless we are supposed to assume that some of these terms are fractions and we are looking for the line's slope at a particular moment it shouldn't be called the calculus of the music market.