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## Integral test for convergence

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# Integral test

## Video transcript

- [Voiceover] Let's explore
a bit the infinite series from n equals one to infinity
of one over n squared. Which of course is equal
to one plus one fourth, that's one over two
squared, plus one over three squared, which is one
ninth, plus one sixteenth and it goes on and on and on forever. So there's a couple of
things that we know about it. The first thing is that all of
the terms here are positive. So all of the terms here are positive. So they're all positive and
that they're decreasing. It looks like they're
decreasing quite quickly here from one to one fourth
to one ninth to one sixteenth, and so they're quickly approaching zero, which makes us feel pretty good that this thing has a chance of converging. And because they're all
positive we know that this sum right over
here, if it does converge is going to be greater than zero. So the only reason why it
wouldn't converge is if somehow it goes unbounded
towards infinity, which we know if this was one over n it would be unbounded towards infinity. So this says that's a possibility here. So if we could show that this is bounded, then that will be a pretty
good argument for why this thing right over here
converges because the only reason why you could
diverge is if you went to either positive infinity
or negative infinity. We already know that this
thing isn't going to go to negative infinity because
it's all positive terms. Or you could diverge if
this thing oscillates, but it's not going to
oscillate because all of these terms are just adding
to the sum, none of them are taking away because none
of these terms are negative. So let's see if we can make
a good argument for why this sum right over here is bounded, especially if we can
come up with the bound, then that's a pretty good argument that this infinite series should converge. And the way that we're going to do that is we're going to explore a related function. So what I wanna do is I wanna explore f of x is equal to one over x squared. You could really view this right over here one over n squared as f of n
if I were to write it this way. So why is this interesting? Well let's graph it. So that's the graph of
y is equal to f of x. And notice this is a
continuous, positive, decreasing function, especially
over the interval that I care about right over here. I guess we could say for
positive values of x, it is a continuous, positive,
decreasing function. And what's interesting is
we can use this as really an underestimate for this
area right over here. What do I mean by that? Well one, this first term right over here, you could view that as the area of this block right over here. That is f of n or I guess
you can say f of one high and one wide, so it's going to be one times one over one squared or one. Let me make sure I'm
using different colors. This term right over
here, that could represent the area of this block,
which is one fourth high and one wide so it is going
to have an area of one fourth. What could this one represent? Well the area of the next
block if we're trying to estimate the area under the curve. And this might look
familiar from when we first got exposed to the integral
or even before we got exposed to the integral and
we were taking Riemann sums. So that right over here, that area, is going to be equal to one ninth. So what's intriguing about
this is we know how to find the exact area, or
the exact area from one to infinity, from x
equals one to infinity. So maybe we can use that somehow. We know what this area is right over here, which we can denote as
the improper integral from one to infinity of f of x dx. We know what that is and I'll
figure it out in a little bit. And if we know what this
is, if we can figure out the value that's going
to be an upper-bound for one fourth plus one ninth plus one sixteenth on and on and on and on. And so that will allow
us to essentially bound what this series evaluates
to and as we said earlier that would be a very good
argument for its convergence. So the whole point here,
I'm not doing a rigorous proof, but really getting
you the underlying conceptual understanding
for a very popular test for convergence or divergence which is called the integral test. Let me just write that
down just so you know what this is kind of the
mental foundations for. So what do I mean here? So let me write this sum again. Let me write it a little bit different. So our original series from n equals one to infinity of one over n squared. It's going to be equal
to this first block, the area of this first
block plus the area of all the rest of the
blocks, the one fourth plus one ninth plus one sixteenth, let me do this in a new color. Which we could write as
the sum from n equals two to infinity of one over n squared. So I just kind of expressed this as a sum of this plus all of that stuff. Now what's interesting
is that this, what I just wrote in this blue
notation that's this block plus this block plus the next block, which is going to be less than this definite integral right over here. This definite integral,
notice it's an underestimate it's always below the
curve, so it's going to be less than that definite integral. So we can write that this
thing is going to be less than one plus instead of writing this I'm gonna write the definite integral. One plus the definite integral from one to infinity of one over x squared dx. Now why is that useful? Well we know how to evaluate
this and I encourage you to review the section on Khan Academy on improper integrals if
this looks unfamiliar, but I'll evaluate this down here. We know that this is the
same thing as the limit as, I'm going to introduce a variable here, t approaches infinity
of the definite integral from one to t of and I'll just write this as x to the negative two dx. Which is equal to the limit
as t approaches infinity of negative x to the
negative one, or actually I could write that as negative one over x. And we're going to evaluate
that at t and at one, which is equal to the limit
as t approaches infinity of negative one over t
and then minus negative one over one so that
would just be plus one. And as t approaches infinity
this term right over here is going to be zero, so this is just going to simplify to one. So this whole thing evaluates to one. So just like that we were able to place an upper-bound on this series. We're able to say that
the series under question or in question, so the
infinite sum from n equals one to infinity of one over
n squared is going to be less than one plus one or it's
going to be less than two. Or another way to think about
it, it's going to be the two is this area, that's one right over there plus this area right over here. So we're saying that this
sum is going to be less than two so we have bounded it above. So we know that it cannot
go to positive infinity. Because all the terms are
positive it's definitely not going to go to negative infinity. And because all the terms
are positive we also know that this isn't going to oscillate between two different values, so
this gives us a pretty good sense that this series converges. And the logic we just
used here to argue for why this converges, once again
not a rigorous proof, but this is the underlying
logic of the integral test.