The Oldest Unsolved Problem in Math

Veritasium
7 Mar 202431:32

Summary

TLDRThe video delves into the enigmatic world of perfect numbers, exploring their properties, history, and the enduring quest to uncover odd perfect numbers. From Euclid's formula to modern computational methods, the narrative traces the evolution of mathematical thought and the persistent allure of this ancient puzzle. Despite the lack of practical applications, the pursuit of perfect numbers underscores the value of pure mathematical curiosity and its potential to yield unforeseen breakthroughs.

Takeaways

  • 📚 The oldest unsolved problem in math is the existence of odd perfect numbers, a mystery that dates back 2000 years.
  • 🧠 Mathematicians have used computers to check numbers up to 10^2200, but have yet to find an odd perfect number.
  • 🌟 A perfect number is defined as a positive integer that is equal to the sum of its proper divisors, excluding itself.
  • 📈 Known perfect numbers follow a pattern: they are all even, and each next perfect number is one digit longer than the previous one.
  • 🔢 Euclid discovered a formula for generating even perfect numbers, but it's unknown if this is the only way to generate them.
  • 🔍 The search for perfect numbers has led to the discovery of Mersenne primes, which are crucial in the search for perfect numbers.
  • 💡 The sigma function, invented by Euler, has been instrumental in understanding the properties of perfect numbers.
  • 📊 Euler's work on perfect numbers led to the Euclid-Euler theorem, which states that every even perfect number has Euclid's form.
  • 🔍 Despite extensive computational efforts, no odd perfect numbers have been found, and there is no proof of their existence.
  • 🤔 The Lenstra and Pomerance Wagstaff conjecture predicts the distribution of Mersenne primes, suggesting there are infinitely many Mersenne primes and, by extension, even perfect numbers.
  • 🌐 The Great Internet Mersenne Prime Search (GIMPS) is a collaborative project that has discovered several new Mersenne primes and perfect numbers.

Q & A

  • What is the oldest unsolved problem in math mentioned in the video?

    -The oldest unsolved problem mentioned is whether any odd perfect numbers exist.

  • What is a perfect number?

    -A perfect number is a positive integer that is equal to the sum of its proper divisors, excluding the number itself.

  • How did Euclid discover a pattern for even perfect numbers?

    -Euclid discovered that even perfect numbers can be generated using the formula 2^(p-1) * (2^p - 1), where p and (2^p - 1) are prime numbers.

  • What is a Mersenne Prime, and how is it related to perfect numbers?

    -A Mersenne Prime is a prime number that can be written in the form 2^p - 1, where p is also a prime number. Mersenne Primes are related to perfect numbers because when 2^p - 1 is prime, the number 2^(p-1) * (2^p - 1) is a perfect number.

  • What is the Great Internet Mersenne Prime Search (GIMPS), and how does it contribute to finding perfect numbers?

    -GIMPS is a collaborative project that uses distributed computing to search for Mersenne Primes. By finding new Mersenne Primes, it indirectly contributes to the discovery of new perfect numbers.

  • What is a 'spoof' in the context of perfect numbers?

    -A 'spoof' is a number that almost satisfies the properties of an odd perfect number but is not actually perfect due to certain factors.

  • What is the current heuristic argument regarding the existence of odd perfect numbers?

    -The heuristic argument suggests that based on the distribution of primes, we should not expect any odd perfect numbers to exist, especially beyond a certain large number.

  • What is the significance of the number 10^2,200 in the search for odd perfect numbers?

    -Researchers have shown that if an odd perfect number exists, it must be larger than 10^2,200, which is the current lower bound for the search.

  • How many known perfect numbers are there, and what are the last two discovered?

    -As of the video's information, there are 51 known perfect numbers. The last two discovered are 2^77,232,917 - 1 and 2^82,589,933 - 1.

  • What is the role of heuristic arguments in mathematics?

    -Heuristic arguments provide a non-proof reasoning based on patterns or probabilities. They can guide mathematicians in their search for solutions but do not constitute a formal proof.

Outlines

00:00

📚 The Enigma of Perfect Numbers

This paragraph delves into the mystery of perfect numbers, which are numbers that equal the sum of their proper divisors, excluding themselves. It discusses the historical pursuit of these numbers by mathematicians, the simplicity and beauty of the problem, and the use of computers to search for them. The video introduces the concept of even perfect numbers, which are all known perfect numbers, and highlights the work of mathematicians like Euclid and Euler in understanding their properties.

05:01

🔍 Euclid's Formula and Mersenne Primes

The second paragraph explains Euclid's formula for generating even perfect numbers, which involves prime numbers and powers of two. It also introduces Mersenne primes, a special class of prime numbers that correspond to the exponents in Euclid's formula. The paragraph discusses the historical journey of discovering perfect numbers, the disproval of Nicomachus's conjectures, and the contributions of mathematicians like Marin Mersenne and Pierre de Fermat to the field.

10:01

💡 Euler's Breakthroughs and the Quest for Odd Perfect Numbers

This paragraph focuses on the significant contributions of Leonhard Euler to the study of perfect numbers. Euler discovered the eighth perfect number and invented the sigma function, which helps in understanding the properties of perfect numbers. He also made strides in proving that every even perfect number follows Euclid's form and attempted to address the existence of odd perfect numbers, although he could not definitively prove or disprove their existence.

15:04

🚀 The Great Internet Mersenne Prime Search (GIMPS)

The fourth paragraph discusses the modern era of perfect number discovery, particularly the establishment of GIMPS, which utilizes distributed computing to search for Mersenne primes. It highlights the rapid advancement in finding larger Mersenne primes and perfect numbers, the challenges involved, and the potential implications for the conjecture of infinitely many perfect numbers. The paragraph also mentions the discovery of the 50th Mersenne Prime and the publication of a book dedicated to the largest known prime number.

20:05

🔍 Spoofs and the Search for Odd Perfect Numbers

This paragraph explores the concept of 'spoofs,' numbers that almost meet the criteria for being odd perfect numbers but fall short due to certain factors. It discusses the efforts of researchers to find properties of spoofs that could rule out the existence of odd perfect numbers. The paragraph also touches on the heuristic argument that suggests the non-existence of odd perfect numbers and the ongoing search for a definitive proof.

25:05

🌟 The Value of Pursuing Mathematical Curiosity

The final paragraph reflects on the importance of studying perfect numbers and other mathematical problems, even if they have no immediate practical applications. It emphasizes the historical shift of seemingly useless mathematical theories into foundational elements of modern technology, such as cryptography. The paragraph encourages curiosity-driven learning and the pursuit of knowledge for its own sake, highlighting the potential for unexpected breakthroughs and applications.

30:06

🎓 Learning with Brilliant

This paragraph serves as a promotional segment for Brilliant, a learning platform that offers a wide range of courses to develop problem-solving skills in various fields. It emphasizes the platform's interactive approach to learning, the ability to set goals and track progress, and the opportunity to apply knowledge to real-world situations. The paragraph concludes with an invitation to try Brilliant for free and a special offer for the first 200 users.

Mindmap

Keywords

💡Perfect Numbers

Perfect numbers are a special class of integers where the sum of all its proper divisors (excluding the number itself) equals the number. For example, 6 is a perfect number because 1, 2, and 3 (its divisors) add up to 6. The video discusses the history and properties of perfect numbers, including their rarity and the challenge of finding them.

💡Mersenne Primes

Mersenne primes are prime numbers that can be written in the form 2^p - 1, where p itself is a prime number. These primes are significant in the context of perfect numbers because they are used to generate even perfect numbers through a formula involving exponentiation. The video mentions the discovery of Mersenne primes and their role in the Great Internet Mersenne Prime Search (GIMPS).

💡Euclid's Formula

Euclid's formula is a method for generating even perfect numbers. It states that if 2^p - 1 is prime, then (2^(p-1))(2^p - 1) is a perfect number. This formula has been used to find all known even perfect numbers, as highlighted in the video, which emphasizes Euclid's contribution to the understanding of perfect numbers.

💡Odd Perfect Numbers

Odd perfect numbers are a hypothetical class of perfect numbers that are not even. The existence of such numbers is an unsolved problem in mathematics, as the video discusses. Despite extensive computational searches, no odd perfect numbers have been found, and there is ongoing debate about their existence.

💡Euler's Sigma Function

The Euler's sigma function is a mathematical tool used to sum the divisors of a number. In the context of perfect numbers, the sigma function of a perfect number always gives twice the number itself. This function is crucial in Euler's work on perfect numbers, as it helps to establish properties and conditions for their existence, as mentioned in the video.

💡Heuristic Arguments

Heuristic arguments are informal reasoning methods used to predict the likelihood of certain mathematical outcomes. In the video, such arguments are used to suggest that there may not be any large perfect numbers, including odd perfect numbers, based on the observed patterns and the rarity of primes.

💡GIMPS (Great Internet Mersenne Prime Search)

GIMPS is a collaborative, distributed computing project that searches for Mersenne primes. By using the collective power of volunteers' computers, GIMPS has discovered several new Mersenne primes, which in turn have led to the discovery of new perfect numbers. The video highlights the success of GIMPS and its role in advancing the search for perfect numbers.

💡Nicomachus's Conjectures

Nicomachus, a Greek philosopher, made several conjectures about perfect numbers, including that all perfect numbers are even and that they end in 6 and 8 alternately. Some of these conjectures have been disproven, while others remain unproven. The video discusses these conjectures and their significance in the study of perfect numbers.

💡Cryptography

Cryptography is the practice of secure communication in the presence of adversaries. The video mentions that number theory, which includes the study of perfect numbers, has found practical applications in cryptography, particularly in the development of secure communication systems that protect sensitive information.

💡Brilliant

Brilliant is a learning platform mentioned in the video as a sponsor. It offers courses in various fields, including math, data science, and programming, to help individuals develop problem-solving skills and follow their intellectual curiosity. The platform is used as an example of how mathematical concepts can be applied in real-world learning scenarios.

Highlights

The oldest unsolved problem in math is about the existence of odd perfect numbers, a mystery dating back 2000 years.

Perfect numbers are rare and their discovery dates back to ancient Greeks, with only a few known, such as 6, 28, 496, and 8,128.

Euclid discovered a pattern for even perfect numbers, which involves consecutive powers of two and prime numbers.

Nicomachus made five conjectures about perfect numbers, including that all perfect numbers are even and end in 6 and 8 alternately.

Mersenne Primes are crucial in finding even perfect numbers, and the search for them has been aided by computer programs.

Leonhard Euler made significant contributions to the understanding of perfect numbers, including the Euclid-Euler theorem.

Euler's sigma function is a powerful tool for analyzing divisors of numbers and has been used to prove properties of perfect numbers.

The Great Internet Mersenne Prime Search (GIMPS) is a collaborative project that has discovered several new Mersenne primes and perfect numbers.

The Lenstra and Pomerance Wagstaff conjecture predicts the distribution of Mersenne primes and supports the idea of infinitely many perfect numbers.

The search for odd perfect numbers has led to the identification of 'spoofs,' numbers that almost meet the criteria for being perfect.

Heuristic arguments suggest that odd perfect numbers may not exist, but these are not proofs and the search continues.

The pursuit of mathematical problems, even those without immediate applications, can lead to significant advancements in fields like cryptography.

The video emphasizes the importance of curiosity-driven learning and the potential for unexpected applications from foundational research.

Brilliant, the video's sponsor, offers a platform for learning and applying mathematical concepts to real-world problems.

Transcripts

00:00

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

00:03

that dates back 2000 years.

00:06

Some of the brightest mathematicians of all time

00:08

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

00:13

In the year 2000

00:14

the Italian mathematician, Piergiorgio Odifreddi,

00:18

listed it among four of the most pressing open problems

00:21

at the time.

00:22

Solving this problem could be as simple

00:24

as finding a single number.

00:26

So mathematicians have used computers

00:28

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

00:33

but so far they've come up empty handed.

00:36

Why do you think this problem has captured the imaginations

00:39

of so many mathematicians?

00:41

- It's old, it's simple,

00:45

it's beautiful.

00:47

- What else could you want?

00:49

So the problem is this.

00:51

Do any odd perfect numbers exist?

00:55

So what is a perfect number?

00:58

Well take the number six for example.

01:00

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

01:05

because that's the number itself,

01:07

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

01:09

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

01:13

which is the number itself.

01:15

So numbers like this are called perfect.

01:18

You can also try this with other numbers like 10.

01:21

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

01:24

If you add those up, you only get eight.

01:26

So 10 is not a perfect number.

01:29

Now you can repeat this for all other numbers,

01:32

and what you find is that most numbers either overshoot

01:35

or undershoot between 1 and a 100,

01:38

only 6 and 28 are perfect numbers.

01:42

Go up to 10,000

01:43

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

01:51

These were the only perfect numbers known

01:53

by the ancient Greeks,

01:55

and they would be the only known ones

01:57

for over a thousand years.

01:59

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

02:02

then we could use that to predict more of them.

02:05

So what do these numbers have in common?

02:08

Well, one thing to notice is

02:09

that each next perfect number is one digit longer

02:13

than the number that came before it.

02:15

Another thing they share is that the ending digit alternates

02:18

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

02:27

But here's where things get really weird.

02:30

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

02:35

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

02:39

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

02:43

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

02:48

of consecutive numbers

02:50

and you can think of each additional number

02:51

as adding a new layer.

02:53

And so these create a triangle,

02:55

which is why these numbers are called triangular numbers.

02:58

Also, every number except for six is the sum

03:01

of consecutive odd cubes.

03:04

So 28 is 1 cubed plus 3 cubed.

03:07

496 is equal

03:09

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

03:13

And 8,128 is equal

03:16

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

03:19

plus 9 cubed all the way up to 15 cubed.

03:22

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

03:25

If you write these numbers in binary,

03:27

six becomes 110,

03:30

and 28 becomes 11100.

03:36

496 becomes

03:38

111110000.

03:42

And 8,128, you guessed it.

03:46

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

03:51

So if you write them out,

03:52

they are all just consecutive powers of two.

04:03

What now around 300 BC

04:05

Euclid was actually thinking along similar lines

04:07

when he discovered the pattern

04:08

that makes these perfect numbers.

04:11

Take the number one and double it,

04:13

you get two now, keep doubling it.

04:15

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

04:20

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

04:22

So 1 plus 2 equals 3.

04:25

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

04:28

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

04:31

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

04:35

Now let's keep doing this.

04:36

Add 1 plus 2 plus 4,

04:38

and you get 7, which is again prime.

04:41

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

04:45

The next perfect number.

04:46

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

04:51

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

04:56

this is prime.

04:57

So you multiply it by 16 and you get 496.

05:01

The third perfect number.

05:03

Now you can keep doing this to find bigger

05:06

and bigger perfect numbers,

05:07

and using this we can rewrite the first three.

05:10

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

05:14

and 28 equals 1 plus 2 plus 4 times 2 squared

05:18

and 496 equals 1 plus 2 plus 4 plus 8

05:22

plus 16 times 2 to the power of 4

05:25

where the first term is prime.

05:27

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

05:30

Take any sum of consecutive powers of 2.

05:33

So 2 to the power of zero which is

05:35

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

05:38

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

05:40

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

05:43

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

05:46

So let's call that T.

05:47

Now multiply this whole equation by two.

05:50

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

05:52

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

05:57

If you now subtract the first equation from the second,

06:00

almost all the terms will cancel out

06:02

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

06:06

So you can replace this whole series

06:09

with one less than the next power of 2.

06:12

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

06:16

28 becomes 2 cubed minus 1 times 2 squared,

06:19

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

06:25

Do you see the pattern?

06:27

This number is always one more than this.

06:29

So if we call this P, then Euclid formula

06:32

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

06:36

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

06:42

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

06:44

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

06:50

Euclid had found a way to generate even perfect numbers,

06:54

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

06:57

So there could be other ways to get perfect numbers,

07:00

including potentially ones that are odd.

07:04

400 years later,

07:05

the Greek philosopher nicomchaus published

07:07

Introdutio Arithmetica, the standard arithmetic text

07:11

for the next thousand years.

07:13

In it, he stated five conjectures statements he believed

07:16

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

07:20

His conjectures were one,

07:23

the nth perfect number has n digits.

07:26

Two, all perfect numbers are even.

07:29

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

07:33

Four, Euclid algorithm produces every even perfect number.

07:37

And five, there are infinitely many perfect numbers.

07:42

For the next thousand years

07:43

no one could prove or disprove any of these conjectures,

07:47

and they were considered facts.

07:51

But in the 13th century,

07:53

Egyptian mathematician Ibn Fallus

07:55

published a list with 10 perfect numbers

07:57

and their values of P.

07:59

Three of these perfect numbers turned out

08:01

not to be perfect at all.

08:03

But the remaining ones are.

08:05

The fifth perfect number is eight digits long,

08:08

which disproves Nicomachus's first conjecture.

08:12

And the next thing to notice is that both the fifth

08:14

and sixth perfect number end in a 6.

08:17

So that disproves Nicomachus's third conjecture

08:19

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

08:23

Two conjectures were proven false.

08:25

But what about the other three?

08:29

Two centuries later, the problem reached Renaissance Europe

08:32

where they rediscovered the fifth, sixth,

08:34

and seventh perfect numbers.

08:37

So far every perfect number had Euclid's form.

08:41

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

08:43

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

08:47

So French polymath Marin Mersenne extensively studied

08:51

numbers of this form.

08:53

In 1644, he published his in a book

08:56

including a list of 11 values

08:58

of P for which he claimed they corresponded to primes.

09:01

Numbers for which this is true

09:03

are now called Mersenne Primes.

09:06

Of his list the first seven exponents of P

09:08

do result in primes

09:10

and they correspond to the first seven perfect numbers.

09:13

But for some of the larger numbers

09:14

like 2 to the 67 minus 1, Mersenne admitted

09:17

to not even checking whether they were prime.

09:20

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

09:24

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

09:30

Mersenne discussed the problem of perfect numbers

09:32

with other luminaries of the time,

09:33

including Pierre de Fermat and Rene Descartes.

09:36

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

09:40

that there are no even perfect numbers

09:42

except those of Euclid.

09:44

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

09:48

it must have a special form.

09:50

It must be the product of a prime

09:52

and the square of a different number.

09:55

If he was right, these would easily have been

09:57

the biggest breakthroughs on the problem since Euclid

09:59

2000 years earlier.

10:01

But Descartes couldn't prove either of those statements.

10:04

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

10:07

real odd perfect numbers.

10:09

But whatever method you use, it takes a long time

10:13

to look for these."

10:15

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

10:18

the Prussian mathematician Christian Goldbach

10:21

met a 20-year-old math prodigy.

10:23

The two stayed in touch corresponding by mail,

10:26

and in 1729, Goldbach introduced this young man

10:29

to the work of Fermat.

10:31

At first, he seemed indifferent,

10:33

but after a little more prodding by Goldbach

10:35

he became passionate about number theory

10:37

and he spent the next 40 years

10:39

working on different problems in the field

10:41

among them was the problem of perfect numbers.

10:45

This Prodigy's name was Leonhard Euler.

10:48

Euler picked up where Descartes had left off,

10:51

but with more success.

10:52

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

10:56

First in 1732,

10:58

he discovered the eighth perfect number, which he had done

11:01

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

11:05

Just as Mersenne had predicted.

11:09

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

11:12

the sigma function.

11:14

All this function does is it takes all the divisors

11:16

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

11:20

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

11:24

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

11:28

And this will be true for all perfect numbers.

11:31

The Sigma function of a perfect number

11:33

will always give twice the number itself

11:35

because the sigma function includes the number

11:37

as one of its divisors.

11:39

Now this may seem like a small change,

11:41

but it ends up being extremely powerful.

11:44

So let's look at a few examples.

11:47

Take a prime number like seven.

11:49

Now, because it's prime,

11:50

you can't rearrange it into a rectangle,

11:53

therefore the only divisors are one and the prime itself.

11:56

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

12:00

Now, to keep things easier to follow,

12:02

we'll just stick to the numbers.

12:04

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

12:07

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

12:10

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

12:14

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

12:17

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

12:21

plus 10 plus 20, which equals 42.

12:25

But you can also write this

12:26

as 1 plus 2 plus 4 times 1 plus 5.

12:30

And this is what really makes

12:31

the sigma function so powerful.

12:33

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

12:36

that don't share factors with each other,

12:37

then you can split up the sigma function

12:39

into the sigma functions of the prime powers

12:41

that make it up.

12:43

So sigma of 2 squared times sigma 5

12:46

is equal to sigma 20.

12:48

And since any number can be written as the product

12:50

of prime powers, you can split up the sigma function

12:53

of any composite number into the sigma functions

12:55

of its prime powers.

12:59

With his new function in hand,

13:01

Euler achieved his second breakthrough

13:02

and did what Descartes couldn't.

13:04

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

13:09

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

13:13

and proved Nicomachus's fourth conjecture.

13:17

Math historian William Dunham called it

13:19

the greatest mathematical collaboration in history.

13:23

But Euler wasn't finished yet.

13:25

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

13:29

So for his third breakthrough, he set out

13:30

to prove Descartes other statement

13:33

that every odd perfect number must have a specific form.

13:37

Because if an odd perfect number does exist,

13:40

you know two things first n is odd.

13:43

And second sigma of n equals 2n.

13:47

Now any number n, you can write as a product

13:49

of different prime numbers

13:51

and each prime can be to some power.

13:54

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

13:57

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

14:02

to their powers, which equals 2n.

14:05

But since all of these factors are primes,

14:07

you can actually split up the sigma function into the sigmas

14:10

of the individual prime powers.

14:12

Now one thing to notice is

14:13

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

14:16

for example seven to the power of 1,

14:18

then the sigma function will be even

14:21

because 1 plus 7 equals 8,

14:23

you'll always get an even number

14:25

because odd plus odd is even

14:28

if the prime number is instead raised

14:29

to an even power like seven squared,

14:33

then the sigma function returns an odd number.

14:36

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

14:40

which equals 57.

14:42

Because odd plus odd plus odd equals odd.

14:46

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

14:48

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

14:52

If instead it's raised to an even power,

14:54

you get an odd number.

14:56

And this is where Euler's genius insight comes in

15:00

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

15:04

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

15:08

Well, what that means is

15:09

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

15:12

because if there were two even numbers,

15:14

you could factor out four.

15:16

But that means you should also be able

15:18

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

15:21

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

15:25

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

15:29

which means that there is exactly one prime

15:32

that is to an odd power

15:33

and all the others must be to an even power

15:36

just as Descartes had predicted.

15:40

Now, Euler refined the form a bit more

15:42

and showed that an odd perfect number must satisfy

15:45

this condition, but even Euler couldn't prove

15:49

whether they existed or not.

15:50

He wrote "Whether there are any odd perfect numbers

15:53

is a most difficult question."

15:56

For the next 150 years

15:58

very little progress was made

16:00

and no new perfect numbers were discovered.

16:04

English mathematician Peter Barlow wrote

16:06

that Euler eighth perfect number "Is the greatest

16:08

that ever will be discovered

16:10

for as they are merely curious without being useful,

16:13

it is not likely that any person will ever attempt

16:16

to find one beyond it."

16:20

But Barlow was wrong.

16:23

Mathematicians kept pursuing these elusive perfect numbers

16:27

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

16:31

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

16:35

So far, Mersenne had done an excellent job.

16:37

He had included Euler's eighth perfect number

16:40

while avoiding others like 29

16:42

that turned out not to lead to a perfect number,

16:45

but 230 years after Mersenne published his list,

16:48

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

16:53

although he was unable to find its factors.

16:57

27 years later, Frank Nelson Cole gave a talk

17:00

to the American mathematical society without saying a word,

17:04

he walked to one side of the blackboard

17:06

and wrote down 2 to the 67 minus 1 equals

17:09

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

17:22

He then walked to the other side of the blackboard

17:24

and multiplied 193,707,721 times

17:30

761,838,257,287

17:36

giving the same answer.

17:38

He sat down without saying a word

17:40

and the audience erupted in applause.

17:44

He later admitted it took him three years

17:46

working on Sundays to solve this.

17:48

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

17:53

From 500 BC

17:54

until 1952 people had discovered just 12 Mersenne primes

17:58

and therefore only 12 perfect numbers.

18:01

The main difficulty was checking whether

18:03

large Mersenne numbers were actually prime.

18:06

But in 1952,

18:08

American mathematician Raphael Robinson wrote

18:10

a computer program to perform this task

18:12

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

18:18

Within 10 months, he found the next five Mersenne primes

18:21

and so corresponding perfect numbers.

18:24

And over the next 50 years,

18:25

new Mersenne primes were discovered in rapid succession,

18:28

all using computers.

18:31

The largest Mersenne prime at the end of 1952

18:34

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

18:38

which is 687 digits long.

18:40

By the end of 1994, the largest Mersenne prime

18:42

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

18:48

which is 258,716 digits long.

18:53

Since these numbers were getting so astronomically large,

18:56

the task of finding numerous end primes became

18:58

more and more difficult even for supercomputers.

19:02

So in 1996,

19:04

computer scientist George Woltman launched

19:06

the Great Internet Mersenne Prime Search or GIMPS.

19:09

GIMPS distributes the work over many computers

19:12

allowing anyone to volunteer their computer power

19:15

to help search for Mersenne primes.

19:17

The project has been highly successful so far,

19:20

having discovered 17 new Mersenne primes,

19:23

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

19:26

And the best part, if your computer discovers

19:29

a new Mersenne prime,

19:30

you'll be listed as its discoverer,

19:33

adding yourself to a list that includes

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