function aliquot_sum(n) 
    n <= 0 && throw(DomainError("Aliquot sum only exists for natural numbers"))
    sum([x for x in 1:div(n, 2) if mod(n, x) == 0])
end

isperfect(n) = aliquot_sum(n) == n
isabundant(n) = aliquot_sum(n) > n
isdeficient(n) = aliquot_sum(n) < n