From The Guardian:
[(Î´U/Î´L) / (Î´U/Î´C) | Sp=0] â‰¤ w – [(Î´U/Î´r) / (Î´U/Î´C) | S = 0]
U is your satisfaction. It’s what you, as a prostitute, care about – the satisfaction you gain from selling your services. Economists like to call it “utility”, which is why they like to use the letter “U”.
L is the amount of leisure you have.
C is the amount of goods and services you, as a consumer, consume.
S is the amount of prostitution you, as a prostitute, sell to your customers.
W is the going price for prostitutes.
R is a measure of your reputation.
(via Cosmic Variance)