they are using this formula: Δwij = η x ai x aj. I think the μ is supposed to be the eta in this case, no idea about the T though. So if you insert the numbers into the formula, you'll get: 0,1x0,5x0,5=0,025. Since we want the new weight we have tu subtract 0,025 from the old wij of 0,5 (which then is 0,475). But this question confused me as well and I'm not entirely sure if the explanation is correct, but this made some sense to me
as i understood it (no guarantee tho): if u think of the original Klondike space, the grey area doesnt have any gold on it so it can't be an oasis. the rarity problem as such i think addresses rather the whole space, because not everywhere is gold. That would leave the plateau, which by definition also says that you dont have any idea where to search next. I guess Isolation could also be applied there, since the shaded area is only very slightly connected to the rest, but I'm not sure. Correct me if I'm worng.