The Oldest Unsolved Problem in Math

- This is a video about the oldest unsolved problem in math

that dates back 2000 years.

Some of the brightest mathematicians of all time

have tried to crack it, but all of them failed.

In the year 2000

the Italian mathematician, Piergiorgio Odifreddi,

listed it among four of the most pressing open problems

at the time.

Solving this problem could be as simple

as finding a single number.

So mathematicians have used computers

and checked numbers up to 10 to the power of 2,200,

but so far they've come up empty handed.

Why do you think this problem has captured the imaginations

of so many mathematicians?

- It's old, it's simple,

it's beautiful.

- What else could you want?

So the problem is this.

Do any odd perfect numbers exist?

So what is a perfect number?

Well take the number six for example.

You can divide it by 1, 2, 3, and 6, but let's ignore 6

because that's the number itself,

and now we're left with just the proper divisors.

If you add them all up, you find that they add to six,

which is the number itself.

So numbers like this are called perfect.

You can also try this with other numbers like 10.

10 has the proper divisors one, two, and five.

If you add those up, you only get eight.

So 10 is not a perfect number.

Now you can repeat this for all other numbers,

and what you find is that most numbers either overshoot

or undershoot between 1 and a 100,

only 6 and 28 are perfect numbers.

Go up to 10,000

and you find the next two perfect numbers 496 and 8,128.

These were the only perfect numbers known

by the ancient Greeks,

and they would be the only known ones

for over a thousand years.

If only we could find a pattern that makes these numbers,

then we could use that to predict more of them.

So what do these numbers have in common?

Well, one thing to notice is

that each next perfect number is one digit longer

than the number that came before it.

Another thing they share is that the ending digit alternates

between 6 and 8, which also means they are all even.

But here's where things get really weird.

You can write 6 as the sum of 1 plus 2 plus 3

and 28 as the sum of one, plus 2, plus 3,

plus 4 plus 5 plus 6 plus 7, and so on

for the others as well, they're all just the sum

of consecutive numbers

and you can think of each additional number

as adding a new layer.

And so these create a triangle,

which is why these numbers are called triangular numbers.

Also, every number except for six is the sum

of consecutive odd cubes.

So 28 is 1 cubed plus 3 cubed.

496 is equal

to 1 cubed plus 3 cubed plus 5 cubed plus 7 cubed.

And 8,128 is equal

to 1 cubed plus 3 cubed plus 5 cubed plus 7 cubed

plus 9 cubed all the way up to 15 cubed.

But here's the one that really blows my mind.

If you write these numbers in binary,

six becomes 110,

and 28 becomes 11100.

496 becomes

111110000.

And 8,128, you guessed it.

It is also a string of ones followed by a series of zeros.

So if you write them out,

they are all just consecutive powers of two.

What now around 300 BC

Euclid was actually thinking along similar lines

when he discovered the pattern

that makes these perfect numbers.

Take the number one and double it,

you get two now, keep doubling it.

You get 4, 8, 16, 32, 64, and so on.

Now starting from one, add the next number to it.

So 1 plus 2 equals 3.

If that adds up to a prime, then you multiply it

by the last number in the sequence to get a perfect number.

So two times three equals six, the first perfect number.

Now let's keep doing this.

Add 1 plus 2 plus 4,

and you get 7, which is again prime.

So multiply it by the last number four, and you get 28.

The next perfect number.

Next, add 1 plus 2 plus 4 plus 8 equals 15,

but 15 isn't prime, so we continue add 16 to get 31,

this is prime.

So you multiply it by 16 and you get 496.

The third perfect number.

Now you can keep doing this to find bigger

and bigger perfect numbers,

and using this we can rewrite the first three.

So 6 equals 1 plus 2 times 2 to the power of 1

and 28 equals 1 plus 2 plus 4 times 2 squared

and 496 equals 1 plus 2 plus 4 plus 8

plus 16 times 2 to the power of 4

where the first term is prime.

But there's a more convenient way to write this still.

Take any sum of consecutive powers of 2.

So 2 to the power of zero which is

1 plus 2 to the 1 plus 2 to the 2,

all the way up to 2 to the n minus 1.

And now because you don't know n, you don't know what

that is equal to, but it will be equal to something.

So let's call that T.

Now multiply this whole equation by two.

So you get 2 to the 1 plus 2 to the 2,

all the way up to 2 to the n, and this is equal to 2T.

If you now subtract the first equation from the second,

almost all the terms will cancel out

and you're left with T equals 2 to the n minus 1.

So you can replace this whole series

with one less than the next power of 2.

So six becomes 2 squared minus 1 times 2 to the 1.

28 becomes 2 cubed minus 1 times 2 squared,

and 496 becomes 2 to the 5 minus 1 times 2 to the 4.

Do you see the pattern?

This number is always one more than this.

So if we call this P, then Euclid formula

that gives a perfect number is 2 to the P minus 1

times 2 to the P minus 1 whenever this is prime.

Now, because you're multiplying it by 2 to the P minus 1,

which is even, this will always give an even number.

Euclid had found a way to generate even perfect numbers,

but he didn't prove that this was the only way.

So there could be other ways to get perfect numbers,

including potentially ones that are odd.

400 years later,

the Greek philosopher nicomchaus published

Introdutio Arithmetica, the standard arithmetic text

for the next thousand years.

In it, he stated five conjectures statements he believed

to be true, but did not bother actually trying to prove.

His conjectures were one,

the nth perfect number has n digits.

Two, all perfect numbers are even.

Three, all perfect numbers end in 6 and 8 alternately.

Four, Euclid algorithm produces every even perfect number.

And five, there are infinitely many perfect numbers.

For the next thousand years

no one could prove or disprove any of these conjectures,

and they were considered facts.

But in the 13th century,

Egyptian mathematician Ibn Fallus

published a list with 10 perfect numbers

and their values of P.

Three of these perfect numbers turned out

not to be perfect at all.

But the remaining ones are.

The fifth perfect number is eight digits long,

which disproves Nicomachus's first conjecture.

And the next thing to notice is that both the fifth

and sixth perfect number end in a 6.

So that disproves Nicomachus's third conjecture

that all perfect numbers end in a 6 or 8 alternately.

Two conjectures were proven false.

But what about the other three?

Two centuries later, the problem reached Renaissance Europe

where they rediscovered the fifth, sixth,

and seventh perfect numbers.

So far every perfect number had Euclid's form.

And the best way to find new ones was by finding the values

of P that make 2 to the P minus 1 prime.

So French polymath Marin Mersenne extensively studied

numbers of this form.

In 1644, he published his in a book

including a list of 11 values

of P for which he claimed they corresponded to primes.

Numbers for which this is true

are now called Mersenne Primes.

Of his list the first seven exponents of P

do result in primes

and they correspond to the first seven perfect numbers.

But for some of the larger numbers

like 2 to the 67 minus 1, Mersenne admitted

to not even checking whether they were prime.

"To tell if a given number of 15 to 20 digits is prime

or not all time would not suffice for the test."

Mersenne discussed the problem of perfect numbers

with other luminaries of the time,

including Pierre de Fermat and Rene Descartes.

In 1638, Descartes wrote to Mersenne, I think I can show

that there are no even perfect numbers

except those of Euclid.

He also believed that if an odd perfect number does exist,

it must have a special form.

It must be the product of a prime

and the square of a different number.

If he was right, these would easily have been

the biggest breakthroughs on the problem since Euclid

2000 years earlier.

But Descartes couldn't prove either of those statements.

Instead, he wrote "As for me, I judge that one can find

real odd perfect numbers.

But whatever method you use, it takes a long time

to look for these."

Around a hundred years later at the St. petersburg Academy,

the Prussian mathematician Christian Goldbach

met a 20-year-old math prodigy.

The two stayed in touch corresponding by mail,

and in 1729, Goldbach introduced this young man

to the work of Fermat.

At first, he seemed indifferent,

but after a little more prodding by Goldbach

he became passionate about number theory

and he spent the next 40 years

working on different problems in the field

among them was the problem of perfect numbers.

This Prodigy's name was Leonhard Euler.

Euler picked up where Descartes had left off,

but with more success.

In doing so, he made three breakthroughs on this problem.

First in 1732,

he discovered the eighth perfect number, which he had done

by verifying that 2 to the 31 minus 1 is prime.

Just as Mersenne had predicted.

For his other two breakthroughs, he invented a new weapon,

the sigma function.

All this function does is it takes all the divisors

of a number, including the number itself and adds them up.

So take any number, say six, sum up all its divisors

and you get 12, which is twice the number we started with.

And this will be true for all perfect numbers.

The Sigma function of a perfect number

will always give twice the number itself

because the sigma function includes the number

as one of its divisors.

Now this may seem like a small change,

but it ends up being extremely powerful.

So let's look at a few examples.

Take a prime number like seven.

Now, because it's prime,

you can't rearrange it into a rectangle,

therefore the only divisors are one and the prime itself.

So Sigma seven is 1 plus 7, which is equal to 8.

Now, to keep things easier to follow,

we'll just stick to the numbers.

But what if instead of seven, you had seven cubed?

Well, again, the sum of the divisors is really simple.

It's just 1 plus 7 plus 7 squared plus 7 cubed.

Now let's use it on a different number, say 20.

The sum of its divisors is 1 plus 2 plus 4 plus 5

plus 10 plus 20, which equals 42.

But you can also write this

as 1 plus 2 plus 4 times 1 plus 5.

And this is what really makes

the sigma function so powerful.

If you have a number that is made up of other numbers

that don't share factors with each other,

then you can split up the sigma function

into the sigma functions of the prime powers

that make it up.

So sigma of 2 squared times sigma 5

is equal to sigma 20.

And since any number can be written as the product

of prime powers, you can split up the sigma function

of any composite number into the sigma functions

of its prime powers.

With his new function in hand,

Euler achieved his second breakthrough

and did what Descartes couldn't.

He proved that every even perfect number has Euclid's form.

This Euclid-Euler theorem solved a 1600-year-old problem

and proved Nicomachus's fourth conjecture.

Math historian William Dunham called it

the greatest mathematical collaboration in history.

But Euler wasn't finished yet.

He also wanted to solve the problem of odd perfect numbers.

So for his third breakthrough, he set out

to prove Descartes other statement

that every odd perfect number must have a specific form.

Because if an odd perfect number does exist,

you know two things first n is odd.

And second sigma of n equals 2n.

Now any number n, you can write as a product

of different prime numbers

and each prime can be to some power.

So let's take that and put it into Euler sigma function.

So you get sigma of n equals sigma of all of those primes

to their powers, which equals 2n.

But since all of these factors are primes,

you can actually split up the sigma function into the sigmas

of the individual prime powers.

Now one thing to notice is

that if you have a prime number raised to an odd power,

for example seven to the power of 1,

then the sigma function will be even

because 1 plus 7 equals 8,

you'll always get an even number

because odd plus odd is even

if the prime number is instead raised

to an even power like seven squared,

then the sigma function returns an odd number.

Sigma of 7 squared equals 1 plus 7 plus 7 squared,

which equals 57.

Because odd plus odd plus odd equals odd.

So if you have the sigma function of an odd prime raised

to an odd power, it will give an even number.

If instead it's raised to an even power,

you get an odd number.

And this is where Euler's genius insight comes in

because here on the right side you've got 2 times n

where n is an odd perfect number, and 2 is even.

Well, what that means is

that on the left side there must only be one even number

because if there were two even numbers,

you could factor out four.

But that means you should also be able

to factor out four on the right side, which you can't

because n is odd and there's only a single 2 here.

So only one of these sigmas here can give an even number,

which means that there is exactly one prime

that is to an odd power

and all the others must be to an even power

just as Descartes had predicted.

Now, Euler refined the form a bit more

and showed that an odd perfect number must satisfy

this condition, but even Euler couldn't prove

whether they existed or not.

He wrote "Whether there are any odd perfect numbers

is a most difficult question."

For the next 150 years

very little progress was made

and no new perfect numbers were discovered.

English mathematician Peter Barlow wrote

that Euler eighth perfect number "Is the greatest

that ever will be discovered

for as they are merely curious without being useful,

it is not likely that any person will ever attempt

to find one beyond it."

But Barlow was wrong.

Mathematicians kept pursuing these elusive perfect numbers

and most started with Mersenne's list of proposed primes.

The next on his list was 2 to the 67 minus 1.

So far, Mersenne had done an excellent job.

He had included Euler's eighth perfect number

while avoiding others like 29

that turned out not to lead to a perfect number,

but 230 years after Mersenne published his list,

Edouard Lucas proved that 2 to the 67 minus 1 was not prime,

although he was unable to find its factors.

27 years later, Frank Nelson Cole gave a talk

to the American mathematical society without saying a word,

he walked to one side of the blackboard

and wrote down 2 to the 67 minus 1 equals

147,573,952,589,676,412,927.

He then walked to the other side of the blackboard

and multiplied 193,707,721 times

761,838,257,287

giving the same answer.

He sat down without saying a word

and the audience erupted in applause.

He later admitted it took him three years

working on Sundays to solve this.

A modern computer could solve this in less than a second.

From 500 BC

until 1952 people had discovered just 12 Mersenne primes

and therefore only 12 perfect numbers.

The main difficulty was checking whether

large Mersenne numbers were actually prime.

But in 1952,

American mathematician Raphael Robinson wrote

a computer program to perform this task

and he ran it on the fastest computer at the time, the SWAC.

Within 10 months, he found the next five Mersenne primes

and so corresponding perfect numbers.

And over the next 50 years,

new Mersenne primes were discovered in rapid succession,

all using computers.

The largest Mersenne prime at the end of 1952

was 2 to the power of 2,281 minus 1,

which is 687 digits long.

By the end of 1994, the largest Mersenne prime

was 2 to the power of 859,433 minus 1,

which is 258,716 digits long.

Since these numbers were getting so astronomically large,

the task of finding numerous end primes became

more and more difficult even for supercomputers.

So in 1996,

computer scientist George Woltman launched

the Great Internet Mersenne Prime Search or GIMPS.

GIMPS distributes the work over many computers

allowing anyone to volunteer their computer power

to help search for Mersenne primes.

The project has been highly successful so far,

having discovered 17 new Mersenne primes,

15 of which were the largest known primes at that time.

And the best part, if your computer discovers

a new Mersenne prime,

you'll be listed as its discoverer,

adding yourself to a list that includes

some of the best mathematicians of all time.

There's even a $250,000 prize

for the first billion-digit prime.

In 2017 Church Deacon John Pace discovered