Problem 27: Find the product of the coefficients, a and b, for the quadratic expression that produces the maximum number of primes for consecutive values of n, starting with n = 0.

This one was straight forward, just had to follow instructions (I didn’t apply any mathematical optimizations, because it looked like if once I got started, I’d be able to solve it all by hand).  I needed to adjust isPrime to return false for values of n less than 2.  I got carried away implementing it SQL style; one long run-on sentence (excluding problem27a, not a single let binding!).  Parallelization improves speed by about 25 percent.

let problem27a =
    seq { for a in -999..999 do for b in -999..999 do yield (a,b) }
    |> PSeq.map(fun (a,b) -> (a*b, Seq.initInfinite id
                                   |> Seq.takeWhile (fun n -> n*n + a*n + b |> isPrime)
                                   |> Seq.length))
    |> PSeq.maxBy snd
    |> fst