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Project Euler Problem #5

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Smallest number divisible by each of 1 to 20:

First cut (takes almost an hour to execute!):

let isFactor n d = n % d = 0

{1..Int32.MaxValue}

|> Seq.find (fun n -> [1..20] |> List.forall (isFactor n))

Second cut (takes a millisecond!):

let rec gcd a b = if b = 0 then a else gcd b (a % b)

[1..20] |> List.reduceBack (fun a b -> (a / (gcd a b)) * b)