Welcome to the first Mathologer video of the year. Today it's about something very
serious and so I'm wearing a totally black t-shirt. You all like Numberphile
right? Me too, except for this one video over there in which they prove the
infamous identity 1+2+3+...=-1/12
using some simple algebra that even kids in primary school should be
able to follow. Since this video was published in 2014 over six million
people have watched it and more than 65,000 have liked it. Unfortunately,
pretty much every single statement made in this video is wrong. And by wrong I
mean wrong in capital letters. In particular, as anybody who knows any
mathematics will confirm 1+2+3+... sums to exactly what common sense
suggests it should namely plus infinity. And this video was not published on the
1st of April. Also, as we all know, the Numberphile videos are presented by
smart guys, in this case university physics professors, who do know their
maths and who are definitely not out to mislead us. So how did they get it so
horribly wrong and what did they really want to say. Well, they started out with
some genuinely deep an amazing connection between 1 + 2 + 3 etc and the
number -1/12 but in the effort to explain this connection in really, really
simple terms they just went overboard and ended up with an explanation that is
not just really simple but also really wrong. Well 6 million views later and the
comment sections of all maths YouTubers are being inundated by confused one plus
two plus three comments that are a direct consequence of this video. For
mathematical public relations it's a disaster. (Marty) It's THE disaster. (Mathologer) Yeah, it's THE disaster. And so I think
it's a good idea to have another really close look at the Numberphile
calculation step by step, state clearly what's wrong with it, how to fix it, and
how to reconnect it to the genuine maths that the Numberphile professors had in
mind originally. Lots of amazing maths look forward: non-standard
summation methods for divergent series, the eta function, a very well-behaved
sister of the Zeta function, the gist of analytic continuation in simple words,
some more of Euler's mathemagical tricks etc. Now, I've tried to make this whole thing
self-contained. So you don't have to have watched my very different other video on
one plus two plus three from over a year ago or anything else to understand this
one. Okay, let's get going. So that we are all on the same page, here real quick is
the whole Numberphile calculation. They call the unknown value of the
infinite series 1 + 2 + 3+... S. As stepping-stones for the calculation they
first calculate the sums of these other two infinite series. So 1-1+1-1+...
and 1-2+3-4+... Adding up the terms of
the first series, we get the partial sums 1. Ok 1 minus 1 is 0, 1 minus 1 plus 1 is
1, 1 minus 1 plus 1 minus 1 is 0 and so on. These partial sums alternate between
0 and 1 and so the Numberphile guys declare that the sum of this infinite
series is CLEARLY the average of 1 and 0 which is 1/2 (Marty) That's not all that clear to me. (Mathologer) Alright we'll get
to that. They also mention that there are other ways to justify this. We'll
also get to that. Ok, second sum. Here they start by considering what happens when
you double this sum. So 2 times S2 is equal to the infinite series added to
itself but now before adding the two infinite series they shift the bottom
series one term to the right. Now 1 plus nothing is 1, minus 2 plus 1 is minus 1, 3
minus 2 is 1, minus 4 plus 3 is minus 1, etc. But that bottom series is the one we
already looked at which, remember, is equal to 1/2 and so ....
Second sum done, great. Now for last sum, that's the one we're really after. Here the Numberphile
guys start by subtracting S2, the sum they just figured out, from S. Now 1
minus 1 is 0, 2 minus minus 2 is plus 4, 3 minus 3 is 0, 4 minus minus 4 is 8, etc.
The zeros don't matter so let's get rid of them. Take out the common factor 4. Ah
the yellow that's our 1+2+3+... sum S again. Now solve for S, and my
usual magic here, and we get -1/12. And here the Numberphile guys take a bow.
But, not so fast! !ll this is really nonsense the way it was presented. In
particular these three identities are false. This means that if on any maths
exam at any university on Earth you're asked to evaluate the sums of these
infinite series and you give the Numberphile identities as your answer
you will receive exactly 0 marks for your answers. It's critical to realise
that in mathematics we have a precise definition which underpins the sum of an
infinite series. Wherever you see infinite series this definition and only
this definition applies unless there are some huge disclaimers to the contrary in
flashing neon lights. Alright, now the Numberphile guys did not include any
such disclaimers and so they too should get 0 marks for their effort. (Marty) Or maybe give them -1/12 marks. (Mathologer) Yeah I
think I can agree with that. OK, what's this definition and what are the
answers that will get you full marks on your maths exam. To evaluate the sum of
an infinite series you calculate the sequence of partial sums
just like the Numberphile guys did at the very beginning. Now if the sequence
of partial sums levels off to a finite number, that is, if the sequence converges,
or if it explodes to plus infinity, or if it explodes to minus
infinity, then this limit is the sum of the infinite series. If no such limit
exists, then the infinite series does not have a sum. That's it, that's the
definition. So for the first Numberphile series the sequence of partial sums
alternates between 0 and 1 and therefore does not have a limit. This means that
this infinite series does not have a sum, neither 1/2 nor anything else. This is
the correct answer for your maths exam. Alright, what about the other two
infinite series? Hmm, well, in the case of 1 plus 2 plus 3 the partial sums
explode to plus infinity and so the sum of the series
is infinity. For the infinite series in the middle the partial sums explode in
size, but neither just to infinity or just to minus infinity, and so this
series also does not have a sum. So these are the answers that get you full marks.
In many ways the most important infinite series are those with a finite
sum which have not featured here yet. So, to give some perspective, here's a
standard example, an infinite geometric series: 1/2+1/4+1/8 and so on.
Now here the partial sums exhibit a really nice pattern and
clearly they converge to 1. (Marty) Yeah, I think this one is clear. (Mathologer) This one is clear and so the sum of this infinite
series is 1. Oh, before I forget, those finite sum series are usually called
convergent series and all the other infinite series are called
divergent series. Keeping this in mind let's have another look at the
Numberphile calculation. Here's the whole thing at a glance. It's just a transcript
of the writing on the brown paper in the Numberphile video. Again, as it was
presented by Numberphile all this is nonsense and worth 0 marks. (Marty) Or less! (Mathologer) Or less :) THIS.
IS. NOT. MATHEMATICS. Don't use it, otherwise you'll burn in mathematical
hell. Having said that there should be some
method to this madness, right? Those guys are smart! But if there is,
then it's clear that the sums you see here cannot possibly represent the usual
sums, as about six million people have been misled to believe by this video. Ok
let's start by doing something that may also seem a little bit crazy. At first
glance, just for fun, and in denial of reality, let's assume for a second that
those three Numberphile series were actually convergent, that is, all had a
finite sum. Then all, ALL highlighted arguments would be valid. This includes
the termwise adding and subtracting of series that was performed here, ... and here, ...
and even the shifting to the right before the addition that a lot of people
view with suspicion. Why would all these operations be ok if we were dealing with
convergent series? Because summation of convergent series is consistent with
termwise addition and subtraction, and shifting. Let me explain that too. There
are differences between finite and infinite sums. For example, infinite
series sometimes don't have a sum whereas finite sums always exist,
rearrangement of the terms can change the sum of convergent infinite series,
etc. On the other hand, the sums of convergent infinite series do share a
lot of the properties of finite sums and it's exactly these properties that make
them so useful. Here the most important three such properties. Let's say you have
two convergent infinite series, okay< with sums A and B. Then, by adding these
two series termwise you get a new infinite series. And now it's quite easy
to prove that this new infinite series is also convergent and that its sum
is equal to A plus B, of course. And the same stays true if you replace all the
pluses with minuses. So termwise addition and subtraction is consistent with
summing convergent series. That's property one. Property two. Multiplying the
terms of a convergent series with sum A by a number, say
five, gives a new infinite series. Again it's really easy to see that the new
series is convergent and that its sum is five times A. So termwise
multiplication by numbers is also consistent with summing convergent
infinite series. That's our second property. Finally, property three. Shifting
the terms of a convergent series with sum A by one term is the same as adding
a zero as the first term to our series, like that. Obviously, the new series is
still convergent and its sum is the same as that of the original series. This also
works the opposite way. Removing a zero term at the front does not change the
sum. Okay, so that's property three. Now we can use these three properties to build
valid arguments, very much like in the Numberphile video. Here's an example. This
is the convergent series that I showed you earlier.
Remember its sum is 1. Now let's say we did not know its sum or even whether
this series is convergent or not. Then we could argue in a legit way like this: Ok,
well we don't know whether it's convergent or not, but if it's convergent
and it's sum is M (M for mystery number :) then because of the number multiplication
property we get that half M is equal to 1/2 times 1/2 which equals to 1/4 plus
1/4 times 1/2 which is equal to 1/8 and so on. But because of the shift property
1/2 M is also equal to this guy here zero plus whatever. Now because of the
addition and subtraction property we are justified to subtract as in the Numberphile
video. So, on the left side we get M minus 1/2 M, that's 1/2 M, and on the
right side we've got 1/2 minus zero that's 1/2, and then everything else kind
of goes away. In total we get M is equal to 1. So our assumption that our
mystery series is convergent with sum M lets us conclude in a valid way
that the only value that M can possibly be is 1. Question: Does this prove that
M is 1? No, and this is very important. Because this argument starts with an
assumption, to be able to conclude that the sum of the series really is 1, you
still have to somehow show that the series we started with is actually
convergent. Hmm so this sort of argument gives you an idea of what to expect but
it does not get you all the way. Anyway let's see what happens when we unleash
this sort of reasoning on the first Numberphile series which we already know
is NOT convergent. Well, so just for kicks, let's assume that the 1-1+1
series was actually convergent with the unknown sum S1. Then, because of the
shift property, S1 would also be equal to 0 plus all the other junk. Again,
because of the addition property, we can add on both sides S1 plus S1 that's
2 S1 and then we add termwise on the right to get, well, 1 there and everything
else cancels out as you can see. And so we get this which is exactly what
Numberphile said S1 should be. So the plot thickens here, right? In fact, in
some ways this line of reasoning would have fitted in better with the rest of
their argument than just plucking the number 1/2 out of thin air. So let's
have another look at the Numberphile argument and let's fit in what we just
did as the first step to justify why S1 should be 1/2. So here we go. This is the
Numberphile argument. Let's round it off by inserting the argument for S1 from
just a moment ago. Here we go. And so here is one way to rephrase the
whole thing to make it into a valid argument. IF, and this is a big, huge
monstrous IF, these three infinite series were convergent, THEN this whole argument
would be valid and the sums of these three infinite series would be exactly the
numbers given by Numberphile. Great, but of course we know that the assumption of
this valid argument is false, that the three infinite series are not convergent.
So, yes, this argument is valid in itself, but what
good is it if the assumption is starts with is false? Well here's an idea.
Since no divergent series has a finite value attached to it, let's dream big.
What if it was possible to extend the notion of summing a
convergent series, what if it was possible to define a super sum. This
super sum should have three key properties: first of all, we don't want to lose
anything so the super sum of a convergent series should be the same as
its normal sum, right? Then all divergent infinite series should be super summable
with finite values. And then the last thing we want is that super summing, just
like normal summing is consistent with adding, shifting, etc. alright. Now such a
super sum extension of the standard infinite sum would be as fantastic as
the extension of the real numbers to the complex numbers, with all sorts of cool
properties for a smaller world remaining true in the bigger world and at the same
time all sorts of new magic appearing in the bigger world. In particular, because
consistency made the argument over there valid for convergent series, we would
expect the argument to still be valid if we replaced ordinary equality by equality
with respect to super summing. Even better, since every infinite series would
have a super sum, we could get rid of the if and so the whole Numberphile video
could be saved by just saying that we are super summing instead of just boring
old summing. What a lovely dream :) (Marty) It's time to wake up! (Mathologer) Yes, sadly. Well, anyway, those of you who watched my
last video on this topic know that there is a super sum. However it only assigns
a sum to some :) divergent series but not to all. In particular, it sums the first
two guys over there. To show how super summing works let's apply it to our first
annoying divergent series. Basically super summing builds upon normal summing
and then averaging out any bouncing around of the
partial sums. We start by calculating the sequence of partial sums. If this
sequence converges, then our super sum equals our normal sum and no tricks are
needed. However if the sequence of partial sums does not converge, as in the
case of this infinite series, then we start with the trickery, building a second
sequence out of the first. The Nth term of this new sequence is just the average
of the first N terms of the first sequence. So, for our particular series
for the first term of the new sequence there's nothing to average, well the
average of 1 is just 1. Ok, then the average of 1 is 0 is 1/2, the average of
1 and 0 and 1 is 2/3, etc. Now, every second number here is 1/2 and the remaining
numbers also converge to 1/2 and this means that overall the second series
converges to 1/2. And that means that our infinite series super sums to 1/2, which
is also the number that Numberphile gets. Now, for other infinite series even the
second sequence may not converge in which case we generate a third sequence,
again by averaging the second sequence. If that doesn't work, then we
generate a fourth, then a fifth, etc. As long as any of those sequences
converges, the infinite series under discussion has the corresponding limit
as super sum. For example, in the case of the second Numberphile series, the first
and second sequences diverge but the third sequence converges to 1/4
which then is our super sum. This is also what Numberphile gets and so everything
is looking good. Until now, now we have to confront the sad truth that for most
infinite series none of the associative averaging sequences of numbers converge
and so these series don't have a super sum. In particular, for 1 plus 2
plus 3 and so on all the partial sums are positive and obviously averaging
over positive numbers will always only result in positive numbers.
In fact, all the associated sequences of numbers will explore to positive
infinity and so 1 + 2 + 3 etc definitely does not super sum to anything finite,
let alone anything finite and negative like -1/12. Returning to the
Numberphile calculation, here's the part that can be totally justified using
super sums instead of regular sums. Because not every divergent series has a
super sum we still need the big IF to make this part of the argument valid. In
itself not too bad, though, since the assumption is actually true, right? And
since super summing is really the most natural extension of normal summing, 1/2
and 1/4 are the only reasonable numbers to associate with the first two series.
Really nice stuff I think. To recap, we now know finite sums, convergen
infinite sums and our new super sums. Oh, by the way, I should mention that in the
literature our super sum would be called generalized Cesaro summation or
generalized Hölder summation. Anyway, these summation methods are proper
extensions of each other and are therefore able to assign meaningful
values to larger and larger classes of series. However, being able to do more
also comes at a price. The more powerful a summation method, the less well behaved
it is. What works for finite sums cannot necessarily be taken for granted for
infinite sums. I already mentioned problems with rearranging convergent series, for
example. Of course, super sums also lack all the nice summy properties that
normal infinite sums lack but they are even summy things that still work for
infinite sums that no longer work for our super sums.
Yes, yes the three basic properties I've been hammering are fine but to assume
that any familiar summy property will also work for super sums in general is
risking zero marks (Marty): Or less! (Mathologer) Or less :) For example, inserting or deleting
infinitely zeros has no effect on convergent series. However, doing the
same to super sums can change things dramatically. For example, if we insert
infinitely many zeros into our first series, like this, the super sum of the
new series will be different from 1. Little puzzle for you:
What's the new super sum? As usual, give your answers in the comments. And this
zero problem is important. I glossed over this because it won't have any bearing
on our discussion, but at some point in the Numberphile calculation they simply
zap infinitely many zeros and this cannot be justified with our three
properties. Bad... The effect of losing more and more properties as you go more and
more general is actually something that you've all encountered before when you
got introduced to larger and larger number systems: fFrst the positive
numbers, then to the integers, the rational numbers, the reals and to the
complex numbers and even beyond to the quarternions and the octonions. Each
time you broaden your world, you lose some nice properties. Second puzzle for
you: Can you think of some properties that get lost along the way as we build
larger and larger number systems? And another puzzle: Suppose we assume that
the 1 + 2 + 3 etc series actually super sums to a finite number, can you
manipulate this identity into a couple of contradictory statements using our
favourite three properties? What can you then conclude from the fact
that you can arrive at contradictory statements? It will be interesting to see
what you can come up with in this respect? Anyway, it's time! (drum roll)
We have to get serious about the connection between our 1 + 2 + 3 series
and - 1/12. So press the pause button, go get your popcorn and your hot
chocolate and let me know when you're back :) (jeopardy music) Ready? Here we go.
Even at the level of super sums we are pretty far removed from what most people
think of as a sum. After all for divergent series, given all the averaging
that is going on, the super summing is really more like finding a super average
than a real summy sum, don't you think? Well it will get more extreme not only in this
respect but also in terms of the maths that is required to understand what is
going on with the -1/12 connection. I'm sure that a lot of you
will already have heard of this connection, so let me just state it first
and then really explain it using the Numberphile calculation as a template.
This is the mega famous Riemann zeta function. It is a function of the complex
variable z. Written as the infinite sum there it makes sense if the real part of
z is greater than 1. However, there is one unique way to extend the Riemann zeta
function to an analytic function for all complex numbers z excepting 1.
Formally, if you substitute z = -1 you get ... Well, of course, minus 1
is not greater than 1, and so we really don't have equality here, and so let's
quickly get rid of their equal sign. Ok, at the same time the right-hand side is
our master villain 1 + 2 + 3 etc and the value of zeta
at minus 1 is equal to you guessed it - 1/12. And this, in a nutshell, is the
genuine, real, actual connection between 1 + 2 + 3 + and - 1/12. But why would
anybody describe this connection as a sum, and what has all this to do wit the
last part of the Numberphile calculation. Well there's more to explain. First,
here's a mini introduction to analytic functions and analytic continuation. This
will be a rough and ready intro which is all we need. You all know what a
polynomial is, right? One of these guys: a constant function or a linear function
or a quadratic function or a cubic, etc. Now let's play a game. Here is a chunk of
a mystery continuous function that is defined for all real numbers. So I'm just
not showing you the part to the left of the y-axis. Here's the question. Just by
looking at this chunk, can you continue the graph and tell me what my function
is? Now you might be tempted to say 'Yes' but the answer is 'No'. There are infinitely
many ways we could continue to the left and here are a couple of examples. Here's one and
there's another one, there's a third one, there's infinitely many different ones.
Were you tricked? (Marty) No. (Mathologer) Sure you were not , but you know the game, right? Of course our
initial chunk is part of a line and it's natural to think of continuing the
function as this same line. But we don't have to. However, if I tell you the
mystery function is linear, then your initial chunk tells you exactly how to
continue the function, there's only one way to continue so that the whole
function is linear. In fact, the same is true if I only told you the function was
a polynomial. Just by looking at the chunk you could be absolutely sure that
my polynomial is linear and exactly which linear function, right? We can now
generalize this simple observation a couple of steps, in a pretty dramatic way
actually. Here we go. First, suppose that our initial chunk is
part of a parabola, or if you like a cubic, or any polynomial. If I then tell
you that my mystery function is a polynomial there's always only going to
be exactly one polynomial that continues our beginning chunk. In other words, a
polynomial is completely determined by any part of it. Second, all we've said
stays true if we think of polynomials as functions of a complex variable and if
you begin with a chunk of the polynomial corresponding to a region in the complex
plane. So on the left, you see the complex number plane where each point stands for
a complex number and I've also colored a small region in
the plane. And so in terms of this picture a polynomial is completely
pinned down by the values it takes on over a region like this. No other
polynomial will take on all the same values there. Again, just relax if all this
seems a little bit too much. Now, the polynomials are the simplest and
most nicely behaved functions but there is a much larger world of functions that
shares a lot of the nicest properties with polynomials. Those are the so-called
analytic functions. These are the complex functions that can be expressed locally
as either regular finite polynomials or as infinite polynomials, so called power
series. For example, the exponential function is an analytic function because
it can be written as an infinite polynomial like this. In fact, pretty much
all our favorite functions such as the trig functions, rational functions, etc.
are analytic. Important for us is that just like a polynomial, an analytic
function is completely pinned down by any initial chunk. So if I give you a
beginning chunk of an analytic function like the exponential function, then no
other analytic function can continue this chunk. This is usually expressed by
saying that analytic continuation of an analytic function is uniquely determined.
In summary, though there may be many nice ad hoc ways to continue an analytic
function there's just one distinguished, most reasonable, absolutely fantastic
never to be improved way to do this, leading to a larger analytic function. Of
course, as I said, this is all very sketchy and you guys in the know will
probably nitpick me to death in the comments. (Marty) Looking forward to it. Ok, in particular I didn't tell
you why we need to drag complex functions into the discussion, but please
just run with it for today and I promise I'll fill in the details soon. In the
meantime you can also read up on things by following the links in the
description. All you really have to remember is this: an initial chunk of an
analytic function nails down the whole analytic function. Now we can join the
dots. We have two completely different notions of best extension. First, for
extending sums to super sums of divergent series
and second for extending a chunk of an analytic function to the whole analytic
function. Combining these two extension ideas, we can finally explain what's
going on with 1 + 2 + 3 + and -1/12. Okay, have a look at this infinite series.
Notice that it's the same as the zeta function except that it includes minuses.
It's also obviously different from the Numberphile series in that it includes a
variable z. So it is actually an infinite family of infinite series, one for each
complex number z. Let's just make a little list of such series corresponding
to a few prominent integer values. For z = 0 we
our 1 minus 1 plus 1 series. Ok for x = - 1 we get 1 minus 2 plus 3
and Mathologer regulars know that for z = 1 and 2 the series are
convergent. Now, in general, these series are convergent for all complex numbers z
in the positive brownish half plane. The infinite series are divergent for all
other z including 1 minus 1 plus 1 etc at 0 and the other guy. However, just like
the two Numberphile series can be super summed, the same is possible for every
z, for all the divergent series in this family. And this allows us to define a
close relative of the zeta function, the so-called
Dirichlet eta function. And this function turns out to be an analytic
function. So to start with, standard summation only gives us part of this
analytic function for which two infinite series converge, this part here. Now, most
mathematicians will simply discount a divergent series that pop up here as
useless artifacts. Instead they will construct the analytic continuation of the
eta function by completely different methods. These methods are very slick and
ingenious. However, they provide very little
intuition and insight into what's really going on here. On the other hand, seeing
that the most reasonable extension of an analytic
function that is defined on part of the complex plane is actually given by the
most reasonable way to assign generalised sums to these supposedly useless
divergent series just feels right to me and leads the way to a more intuitive
understanding of analytic continuation, at least in this case. But now here's a
great thought: we just used a generalised sum to construct an analytic
continuation, right? Let's turn the idea around, let's use analytic continuation
to identify candidates for a generalised summation method. And this is exactly
what happens in the case of 1 + 2 + 3 etc and - 1/12 and the zeta function. You
get the zeta function when you replace all the minuses in the eta function by
pluses. Well you get part of it, the brownish part. which is the part of the
complex plane for which the infinite series on the right converge. For all
other z the resulting infinite series are divergent and even super summing
doesn't help for the white part on the left. So the super summing trick for
eta just doesn't work for zeta. The trick to use is encoded in the finale of
the Numberphile pseudo proof. That's the brownish bit down there. Remember this
part of the argument takes the sum S2 of the 1 - 2 + 3 series and spits out a
sum for the 1 + 2 + 3 series formally these two sums are just what you get
when you let z equal to -1 in the infinite series of the eta and zeta
functions and actually the Numberphile calculation is just a special instance
of a calculation involving eta and zeta. Let's be brave. Ignoring questions
of legality, let's unleash exactly the same calculation on eta and zeta. So
instead of subtracting S2 from S, let's subtract eta
from zeta. Ready? ... Right, take out the common factor down there. That's zeta again in
the brackets. Now let's solve for zeta. There my magic
again okay that's really exactly the same as the last part of the Numberphile
calculation using zetas and eta instead of the Numberphile series. Just as a
check after substituting z = -1 this identity turns into
this, and with S2 being 1/4 we get this. Okay more magic and we're back to
-1/12. But didn't we say that the Numberphile computation was nonsense?
(Marty) Yes, we did. (Mathologer) We definitely did. And it is, but some magic happens with zetas and eta which
saves our zetas eta calculation from being nonsense and that is the magic of
analytic continuation. Both the series for zetas and eta are convergent for
every z in this brownish region. This means that for these values of z our
calculation and the resulting equation above are pure, correct, 100% approved
bona fide mathematics. But, as well, eta is defined and analytic for all z
and the same is true for the denominator. But then the right side, as a quotient
of two functions that are analytic everywhere, is itself defined and
analytic everywhere except possibly at the zeroes of the denominator. In fact, a
closer look reveals that the whole right-hand side is analytic everywhere
except at z =0. Here comes the punchline and this punchline
hinges on the chunks-pin-down-analytic-functions business that I've been going
on about. You should really try to understand this. Okay, so both the left
side and the right side are analytic in this part of the complex plane here. But
since the right side is analytic everywhere, because of our chunks-pin-
down-analytic-functions property, the right side has to be equal to the
elusive analytic continuation of the zeta function on the left that everybody is
really interested, the analytic continuation of the zetas function. So
this identity is a real jewel as it gives an explicit way to calculate any
value of the Riemann zeta function, analytic continuation and all via the eta
function which, remember, we defined via super summing. In other words, it actually
makes sense to use this identity as a definition for the zeta function which
works for all z. For example, setting z=0 we get this one here
and eta of zero was just the super sum of 1 minus 1 plus 1 etc which is equal to
1/2 and so zeta of 0 is equal to -1/2. Here just a couple more interesting
values for zeta. Nice. The zeroes are particularly interesting. In fact, it
turns out that zetas has zeros at all negative even integers, -2, -4,
-6, and so on. These are the so-called trivial zeroes of the zeta
function. I'm sure if you made it this far you've
also heard of the Riemann hypothesis which is all about other zeros of the
zeta function. It says that all other zeros are situated on this blue line
here and what's also really interesting is that on the right hand side we
actually don't have to super sum to calculate the values of zetas on the
blue line because just with ordinary summing eta can be evaluated
everywhere in this part of the complex plane, this part here which includes this
critical line. Lots and lots of other interesting things one could say here
but, well, we are here to wrap up this whole 1 plus 2 plus 3 is equal to
-1/12 business. Alright, ok, let's do it. In the first instance it is really our
identity up there that the Numberphile video is attempting to capture and it's
definitely tempting to express his identity as a new generalized sum
maybe like this. I've decorated the equal sign with an R in honour of Ramanujan who
seems to have been the first to think this way (NOT Euler as many people think).
In fact, in Ramanujan's notebook we can find a calculation very similar to the
one that's in the Numberphile video. Of course, there's a huge difference between
a monumental genius quickly abbreviating all his
complicated stuff in a personal note and a YouTube video addressed to a general
audience, right? What you see up there is part of a general method called
Ramanujan summation that assigns values to all sorts of divergent series
including the three Numberphile series. An important aspect of infinite series
which is often overlooked is the order in which the terms are summed. We are not
just adding an infinite set we are doing so in a certain order and this has all
sorts of important implications. So, for example, no matter how we arrange the
natural numbers into an infinite series this infinite series will always diverge
to plus infinity using standard summation. However, how exactly, how fast,
slow, regular, erratic it diverges to infinity depends very much on the order
of the terms in the series. The Ramanujan sum lacks pretty much all the nice
summy properties that we encountered today. However, it manages to capture
aspects of naturally ordered series and it pops up in many other branches of
the theory of divergent series in addition to the one we talked about
today. Check out some of the links in the description, especially if you know
a lot of maths the article by Terry Tao. Also I'm planning another video on the
so called Euler-Maclaurin summation formula which establishes a powerful
connection between sums and integrals and which is the starting point for
Ramanujan's sum. Just to whet your appetite here's one closely related
-1/12 fact. The nth partial sum of our infinite series is N(N+1)/2
So, if we plug in 1, 2, 3, 4 etc. for N the formula will spit out those partial
sums 1, 3, 6, 10. Now let's replace N by x and graph the resulting function. That's
a quadratic with zeros at 0 and - 1. Now the remarkable thing about this
graph is that the signed area here is equal to -1/12. And this is
definitely no coincidence. Really amazing stuff, don't you agree? And that's it,
finally, for today (and now I go and kill myself :) and I promise
the next video will be a LOT shorter.
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