king Equations may involve derivatives of functions.

In an article on interest rate deregulation, Christofi and Agapos$1$

solve the equation

r D

$1$ C

rL

dC

dD

for $($the Greek letter $$eta$).$ Here r is the deposit rate paid by

commercial banks, rL is the rate earned by commercial banks,

C is the administrative cost of transforming deposits into

return$-$earning assets, D is the savings deposits level, and is

the deposit elasticity with respect to the deposit rate.