Problem Statement

Write a function which says whether a number is perfect.

A perfect number is a positive integer that is the sum of its proper positive divisors excluding the number itself. Equivalently, a perfect number is a number that is half the sum of all of its positive divisors (including itself).

Note:   The faster   Lucas-Lehmer test   is used to find primes of the form   2n-1,   all known perfect numbers can be derived from these primes using the formula   (2n - 1) × 2n - 1. It is not known if there are any odd perfect numbers (any that exist are larger than 102000). The number of   known   perfect numbers is   51   (as of December, 2018),   and the largest known perfect number contains  49,724,095  decimal digits.

Let's start with the solution:

let perf n =
  let sum = ref 0 in
    for i = 1 to n-1 do
      if n mod i = 0 then
        sum := !sum + i
    done;
    !sum = n


(* range operator *)
let rec (--) a b =
  if a > b then
    []
  else
    a :: (a+1) -- b

let perf n = n = List.fold_left (+) 0 (List.filter (fun i -> n mod i = 0) (1 -- (n-1)))