A set A has an accumulation point p if for every 𝜀>0 there is an x∈A with x ne p and |x-p|<𝜀. Informally, p is an accumulation point of A if there are points of A that are arbitrarily close to p. Note that the fact that p is an accumulation point of the set A has nothing to do with whether p is actually an element of A. For example, the set A=\vertn∈ℕ} has one accumulation point, 0, because for every 𝜀>0 there is an n∈ℕ with 1/n<𝜀. Here the accumulation point 0 is not an element of the set A.

They choose from a menu of regular challenges, superchallenges, or megachallenges.

This fraction of grain offal contains a number of over-lapping sub-fractions, which are known in the trade as red-dog, shorts, middlings, and bran. A portion of the red-dog is contained in the lowest grade of straight flour.

Roger was in the library, trying to choose, from an embarrassment of riches, the ten of his father's books which he was to be permitted to take to the city.