User:WillNess

I'm interested in Haskell.

I like this:

--   inifinte folding idea due to Richard Bird
--   double staged production idea due to Melissa O'Neill
--   tree folding idea Dave Bayer / simplified formulation Will Ness
primes = 2 : g (fix g) 
  where                
    g xs = 3 : gaps 5 (foldi (\(c:cs) -> (c:) . union cs) 
                             [[x*x, x*x+2*x..] | x <- xs])
    gaps k s@(c:t)                                        
       | k < c = k : gaps (k+2) s     -- minus [k,k+2..] (c:t), k<=c
       | True  =     gaps (k+2) t     --   fused to avoid a space leak

fix g = xs where xs = g xs            -- global defn to avoid space leak

foldi is on Tree-like folds page. union and more at Prime numbers.

The math formula for Sieve of Eratosthenes,

$\textstyle \mathbb {S} =\mathbb {N} _{2}\setminus \bigcup _{p\in \mathbb {S} }\{np:n\in \mathbb {N} _{p}\}$

where

$\textstyle \mathbb {N} _{k}=\{n\in \mathbb {N} :n\geq k\}$   . . . or,  $\textstyle \mathbb {N} _{k}=\{k\}\bigcup \mathbb {N} _{k+1}$   :) :) .

Trial division sieve:

$\textstyle \mathbb {T} =\{n\in \mathbb {N} _{2}:(\not \exists p\in \mathbb {T} )(p\leq {\sqrt {n}}\,,p\mid n)\}$

If you're put off by self-referentiality, just replace $\mathbb {S}$ or $\mathbb {T}$ on the right-hand side of equations with $\mathbb {N} _{2}$.