Ah that vaguly looks familar (high school was a long time ago). Now I don't think it can be applied to this series as part of the definition says "where no two xj are the same." Or am I misssing something?
Yes, you are missing something :-)
In this case, the x's are 1,2,3,4,5,...,n and the y's are 2,2,2,3,7,...m
And the method gives a function f so that f(1) = 2, f(2) = 2, f(3) = 2, f(4) = 3, and so on...