## 2012-03-03

### Riemann hypothesis, DIY

Continuing with useless exercises, let's look at prime numbers. We shall use Haskell this time.

```-- prime numbers: [2,3,5,7,11,13,17,19, ...]
pr :: [Int]
pr = [p | p <- [2 .. ], maximum [d | d <- [1 .. p - 1], p `mod` d == 0] == 1]
```

The first line, starting with '--' is a comment. The second line declares 'pr' to have type "list of Int". Typings are optional in Haskell. The last line, uses "list comprehension" (yes, it is related to "set comprehension" used in one of the recent posts) to define 'pr' as those elements of the infinite list [2, 3, 4, 5, ...] (denoted [2 ...]), whose maximal proper divisor is 1. This is a very inefficient, quadratic way to build the list of all primes.

In addition, define a list of all twin primes:

```-- twin primes, primes whose difference is 2
twinp :: [Int]
twinp = [n | n <- pr, (n + 2) `elem` (take (n + 4) pr)]
```

"x `elem` s" is true iff x is en element of list s. Ugly '(n + 2) `elem` (take (n + 4) pr)' is needed because for an infinite s, "x `elem` s" either returns True or never returns: without some additional knowledge about the list, the only way to determine whether x belongs to it is by linear search, which might never terminate. In our case, 'pr' is strictly ascending and hence the search can be limited to some finite prefix of the list ((take n s) returns first n elements of s). Perhaps there is a way to specify list properties that `elem` can rely on, I don't know, I opened the Haskell specification an only hour ago.

Let's do some "visualisation":

```-- an (infinite) string containing a '+' for each element of s and '.' for each
-- element not in s, assuming that s is ascending and s !! n > n.
-- dot pr == "..++.+.+...+.+.. ..."
dot :: [Int] -> [Char]
dot s = [ if (x `elem` (take (x + 2) s)) then '+' else '.' | x <- [0 ..]]
```

For example, (dot pr) is an infinite string containing "+" for each prime number and "." for each composite.

By printing (dot s) in rows, k columns each, some hopefully interesting information about distribution of elements of s can be obtained. First, split s into sub-lists of length k:

```-- (dot s), sliced into sub-strings of length k, separated by newlines
sl :: [Int] -> Int -> [[Char]]
sl s k = ['\n' : [ (dot s) !! i | i <- [j*k .. (j+1)*k - 1]] | j <- [0 ..]]
```

then, concatenate them all:

```-- concatenation of slices
outp :: [Int] -> Int -> [Char]
outp s k = concat (sl s k)
```

A few examples:

```putStr (outp pr 6)

..++.+
.+...+
.+...+
.+...+
.....+
.+....
.+...+
.+...+
.....+
.....+
.+....
.+...+```

immediately one sees that, with the exception 2 and 3, prime numbers have remainder 1 or 5 when divided by 6. For 7, the picture is

```putStr (outp pr 7)

..++.+.
+...+.+
...+.+.
..+....
.+.+...
..+...+
.+...+.
....+..
...+.+.
....+..
.+.+...
..+...+
.....+.
......+
...+.+.
..+.+..
.+.....
.......
.+...+.
....+.+
.......
..+.+..
...+...
..+...+```

it's easy to see that "vertical" lines of "+", corresponding to a particular remainder are now (unsurprisingly) replaced with slopes. By changing the width of the picture, one can see how regular pattern changes its shape periodically
```putStr (outp pr 23)

..++.+.+...+.+...+.+...
+.....+.+.....+...+.+..
.+.....+.....+.+.....+.
..+.+.....+...+.....+..
.....+...+.+...+.+...+.
............+...+.....+
.+.........+.+.....+...
..+...+.....+.....+.+..
.......+.+...+.+.......
....+...........+...+.+
...+.....+.+.........+.
....+.....+.....+.+....
.+...+.+.........+.....
........+...+.+...+....
.........+.....+.......
..+.+...+.....+.......+
.....+.....+...+.....+.
......+...+.......+....
.....+.+.........+.+...
..+...+.....+.......+..
.+.+...+...........+...```

```putStr (outp pr 24)

..++.+.+...+.+...+.+...+
.....+.+.....+...+.+...+
.....+.....+.+.....+...+
.+.....+...+.....+......
.+...+.+...+.+...+......
.......+...+.....+.+....
.....+.+.....+.....+...+
.....+.....+.+.........+
.+...+.+...........+....
.......+...+.+...+.....+
.+.........+.....+.....+
.....+.+.....+...+.+....
.....+.............+...+
.+...+.............+....
.+.........+.+...+.....+
.......+.....+.....+...+
.....+.......+...+......
.+.........+.+.........+
.+.....+...+.....+......
.+...+.+...+...........+
.......+...+.......+...+```

One of the more striking examples is with twin primes:
```putStr (outp twinp 6)

...+.+
.....+
.....+
......
.....+
......
.....+
......
......
.....+
......
.....+
......
......
......
......
.....+
.....+
......
......```

Every pair of twin primes except for (3, 5) has a form (6*n - 1, 6*n + 1).

As Gauss reportedly quipped (when refusing to work on the last Fermat's theorem): "number theory is full of problems whose difficulty is only surpassed by their uselessness".