gistfile1.fs

let PeriodValue payment rate period = 
    payment / ((1.0 + rate) ** period)    

let PresentValue (factors: AnnuityFactors) =     
    [1 .. factors.Periods]
    |> List.map (fun period -> PeriodValue factors.Payment factors.InterestRate (float period))
    |> List.sum 

You can also make this lazy, though that would only make a difference for large values of Periods:

let PresentValue (factors: AnnuityFactors) =
    seq { 1 .. factors.Periods }
    |> Seq.map (float >> PeriodValue factors.Payment factors.InterestRate)
    |> Seq.sum