You can use Define + New Definition to declare an expression of the form f (x) to be a function without specifying any of the function values or behavior. Thus you can use the function name as input when defining other functions or performing various operations on the function.
You can use Define + New Definition to declare a character to be a (generic) constant.
Define + New Definition
a
Calculus + Implicit Differentiation
ax + y = 0 (Differentiation Variable: x) Derivative: a + y′ = 0
bx + y = 0 (Differentiation Variable: x) Derivative: b′x + b + y′ = 0
Note that a′ = 0 while b′ is not automatically assumed to be 0.