let sumOfDivisors n = let rec loop current max acc = if current > max then acc else if n % current = 0 then loop (current + 1) max (acc + current) else loop (current + 1) max acc let start = 2 let max = n / 2 (* largest factor, apart from n, cannot be > n / 2 *) let minSum = 1 + n (* 1 and n are already factors of n *) loop start max minSum printfn "%d" (sumOfDivisors 10) (* prints 18, because the sum of 10's divisors is 1 + 2 + 5 + 10 = 18 *)