Last summer, Math With Bad Drawings had a nice series on the notion of infinity that I recommend highly. This topic is a perennial struggle for math majors to grasp, and I like the approach that the author uses to sell this difficult notion.
diverges. This is a perennial head-scratcher for students, as the terms become smaller and smaller yet the infinite series diverges.
To show this, notice that
and so on. Therefore,
and, in general,
Since , we can conclude that the harmonic series diverges.
However, here’s an amazing fact which I hadn’t known before the Math With Bad Drawings post: if you eliminate from the harmonic series all of the fractions whose denominator contains a 9, then the new series converges!
I’ll discuss the proof of this fact in tomorrow’s post. Until then, here’s a copy of the comic used in the Math With Bad Drawings post.