• How to handle multiple input variables? If there are multiple input variables, say M and N it is not obvious how to order complexity classes. Is O(M N²) better or O(M² N)? If one variable is bounded by a function of the other, say M ∈ O(√N) all M can simply be replaced with the corresponding function of N to obtain a complexity class of a single variable. If both inputs are independent of each other, some other order has to be defined. One could, for instance, replace all M and N with a single variable K to get a preliminary order. If the OP then specifies whether M or N dominates, ties could be broken by the highest-order discrepancy in favour of the smaller variable.