Sony PS3 catching up to Wii in Japanese sales

[alephnull]

If d(PS3 sales)/dt > d(Wii sales)/dt for all t>t0 as t->inf, l'Hopital's rule implies that at some point total PS3 sales > total Wii sales. What is your definition of "catching up"?

What will prevent this is the derivative of PS3 sales eventually being <= the derivative of wii sales.