Question

How to tell if a function is linear

Answer 1

if a function is linear, itll have these properties:

f(x+y) = f(x)+f(y)

f(kx) = k f(x) for some constant k

and f(0) = 0

Answer 2

some texts muddy the waters by calling affine functions linear as well; an affine function is a linear function plus a constant

Answer 3

...plus a non zero constant :)