I found these oscillator configurations(all are similar) at various sites on the web, but here and here the author says its a negative resistance oscillator. It's a popular oscillator among amateur radio enthusiasts due to its ability to oscillate at a wide range of frequencies, its often called emitter coupled oscillator/differential pair oscillator etc. Such one is even briefly touched on the book Experimental Methods in RF Design
My question is that, is these circuits negative resistance oscillators, if so how is negative resistance achieved in these circuits? If not, how does such circuits oscillate?
EDIT: I found another different circuit that is said to be negative oscillator too. I would like to add it to the question too. The page also contains several other circuits similar to the ones shown above.