# Goldbach’s conjecture, Lagrange’s theorem, and 2019

The previous post showed how to find all groups whose order is a product of two primes using 2019 as an example. Here are a couple more observations along the same line, illustrating the Odd Goldbach Conjecture and Lagrange’s four square theorem with 2019.

## Odd Goldbach Conjecture

Goldbach’s Conjecture says that every even number greater than 2 can be written as the sum of two primes. The Odd Goldbach Conjecture, a.k.a. the Weak Goldbach Conjecture, says that every odd number greater than 5 can be written as the sum of three primes. The Odd Goldbach Conjecture isn’t really a conjecture anymore because Harald Helfgott proved it in 2013.

So it should be possible to write 2019 as the sum of three primes. In fact there are 2,259 ways to write 2019 as a non-decreasing sequence of primes.

```      3 +   5 + 2011
3 +  13 + 2003
3 +  17 + 1999
...
659 + 659 +  701
659 + 677 +  701
673 + 673 +  673
```

## Lagrange’s four square theorem

Lagrange’s four square theorem says that every non-negative integer can be written as the sum of four squares. 2019 is a non-negative integer, so it can be written as the sum of four squares. In fact there are 66 ways to write 2019 as a sum of four squares.

```     0  1 13 43
0  5 25 37
0  7 11 43
...
16 19 21 31
17 23 24 25
19 20 23 27
```

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