This conjecture is one of the major unsolved problems in number theory, being one of the four problems that [Landau in 1912](https://en.wikipedia.org/w/index.php?title=Landau%27s_problems&oldid=869848981) described as “unattackable at the present state of mathematics”. (All four problems [remain open](https://terrytao.wordpress.com/2014/11/13/254a-announcement-analytic-prime-number-theory/).) Goldbach is right here about Euler's intuition being mistaken: the fact that the density of numbers of the form $a^2 + 1$ is less than that of the prime numbers is not relevant to the question of whether there are infinitely many of them. The portrait on the right is certainly no Goldbach. It's evident from his dress first and foremost. All the arguments are well summarized here: https://hsm.stackexchange.com/a/559/8163 The internet for some reason thinks that it's Goldbach. The first series has denominators of the form 4$k$-1, where 4$k$-1 isn't prime. The second series has denominators of the form 4$k$+1, where 4$k$+1 isn't prime. Where $4k-1$ (resp. $4k+1$) is prime? It is just in the last paragraph that Goldbach mentions his conjecture: every odd integer can be written in the form $p + 2a^2$, where $p$ is a prime (or 1, then considered a pime) and $a\geq0$ is an integer. In his reply in December that year, Euler wrote back saying that he had verified this up to 1000 and later he extended the verification up to 2500. Euler also mentioned that as the odd number $2n + 1$ increased, the number of representations tended to increase. It was only more than a century later, in 1856, that Moritz Stern,a mathematician at the university of Gottingen, was able to test the conjecture up to 9000 with the help of his students. At the time, a lot of primality testing (checking if a number is prime) was done with exhaustive divisibilty testing ([sieve of Erastothenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)) done by hand, thus taking a long time. Stern was able to find 2 exceptions: 5777 and 5993, that to this day are the only known exceptions to Goldbach's conjecture. It's interesting to note that with today's available computing power anyone can easily find the exceptions in a fraction of a second using a regular computer. Below is the code you can use to find the exceptions: ```C private bool isTwiceSquare(long number) { double squareTest = Math.Sqrt(number/2); return squareTest == ((int)squareTest); } ``` ```C int[] primeList = ESieve(10000); int result = 1; bool notFound = true; while(notFound){ result += 2; int j = 0; notFound = false; while (result >= primeList[j]) { if(isTwiceSquare(result-primeList[j])){ notFound = true; break; } j++; } } ``` Complete source code [here](https://www.mathblog.dk/files/euler/Problem46.cs) ## Historical Context Euler is generally known as one of the greatest mathematicians of his time, with a vast body of work and extended contributions in math and physics. Christian Goldbach, on the other hand, is known by most people because of one single idea: the Goldbach Conjecture - a statement in number theory which is very easy to understand – every even integer greater than 2 is the sum of two prime numbers - but still unproved more than 250 years after he first wrote it down in 1742.  Goldbach was born in Germany in 1690, 17 years before Euler, and was often unfairly described as an "amateur" mathematician or a mathematician who chose to pursue a professional career in law. The relationship between Euler and Goldbach begins in 1727 in St. Petersburg: Euler, 20 years old at the time, had just gotten a position in the Petersburg Academy and was eager to prove his talent, while 37 year old Goldbach was responsible for supervising the young Tsar's education. Since then, Euler and Goldbach corresponded frequently through letters almost until Goldbach's death in 1764 (196 letters in total). These letters constitute an incredible opportunity to delve into Euler and Goldbach's lives: from Euler's rise in the mathematical world to Goldbach's explorations that lead to several of his theorems and conjectures,as well as their own personal and professional struggles (if you want to read the letters they have been translated to English and edited in this [book](https://www.springer.com/gp/book/9783034808927)). The letter you are reading now was written by Goldbach in 1752 (after his most famous conjecture), where Goldbach expressed another of his conjectures: he believed that every positive odd integer is either prime or the sum of a prime and twice a square. In the following comments we are going to test the conjecture and explore the history that came after. If you want to read the original version of the letter in German you can have access to it [here](http://eulerarchive.maa.org/correspondence/letters/OO0878.pdf). After Stern's find there was a question left unanswered: is there a finite number of exceptions to Goldbach's conjecture? No one has been able to prove this but there are a few things that might indicate that the number of exceptions is indeed finite, one of them being the fact that the average number of ways of expressing an odd integer $2n - $1 as a prime plus twice a square increases without limit as $n$ increases. Actually it is not difficult to see that if the number of primes up to $n$ is on the order of $\frac{n}{\log n}$ while the number of squares up to $n$ is on the order of $n^{1/2}$, the average number of ways of expressing odd numbers up to $2n-1$ as a prime plus twice a square is at least $$ n^{1/2}\frac{n}{\log n}\frac{1}{2n-1} \approx \frac{n^{1/2}}{\log n} $$