let rec Lucas x =
    match x with
    | x when x <= 0 -> failwith "Errore! x <= 0"
    | 1 -> 1
    | 2 -> 3
    | x -> Lucas (x - 1) + Lucas (x - 2)

[<EntryPoint>]
let main argv =
    printfn "Lucas (2) = %i" (Lucas 2)
    printfn "Lucas (3) = %i" (Lucas 3)
    printfn "Lucas (5) = %i" (Lucas 5)
    0