Hi, guys......Sorry, but some of you are making a terrible mess out of something that's relatively simple from a math POV, although the math may seem hairy for some. I'll also state clearly that I have no intentions of being condecending here- I've been teaching radio theory at polytecnic level for several years, and seen quite alot of students struggling with this.....
Harmonic and intermodulation are the two sides of the same coin - harmonic distortion is a single frequency item - injecting F1 results if F1+n x F1, where n is an integer.
Intermodulatioin is usually described with two frequencies - F1 and F2, resulting in intermodulation products ( n x F1 +- m x F2).
Ex. above with 100Hz and 1000Hz gives 900,1100,1900,2100,1200, 800, 1800, and so on, all depending on the nonlinearites in the transfer function.
The optimum transfer function is Aout=k x sin(wt), where k is your amplification factor -graphically displayed as a straight line where the rise angle coefficient is the Amp factor.
The problems arise when the curve deviates from the straight line, introducing higher order factors - sin^2, sin^3 and so on.
These are the factors responsible for the higher order distortion products. If only sin^2 were present, there would only be 1 order products 2 x F1 and F1 +- 2 x F2.
The reference to SSB modulation, in essence AM and all its derivates, ( SSB being one of them), is a situation where you strive to achieve a perfectly quadratic transfer function based on sin^2. In the frequency mixer circuits used for this purpose, one actually uses purposely developed diodes wiith a well controlled dforward current to optimize the 2nd order behaviour - nothing but well controlled IMD.
Mingling these processes with clipping, noise, oscillation etc. is trying to describe the interaction of highly different processes - truly a daunting task from a theoreticacl POV.