One reason that we converted the basic production function as specified by Robert Solow, Y = F(K,L) to the form y = f(k), where y and k represent Y/L and K/L, respectively, is that this form:
a. uses lower case letters, which are less confusing in written sentences.
b. matches our definition of economic growth as the change in per capita output.
c. is easier to write because there is only one factor, rather than two, in the production function.
d. is closer to the production function suggested by Adam Smith.