The fact that a unique sequence so corresponds to any number "N" was observed by D. H. Lehmer. Indeed a greedy algorithm finds the "k"-combination corresponding to "N": take "c""k" maximal with formula_3, then take "c""k" − 1 maximal with formula_4, and so forth. Finding the number "N", using the formula above, from the "k"-combination ("c""k"...,"c"2,"c"1) is also known as "ranking", and the opposite operation (given by the greedy algorithm) as "unranking"; these operations are known by those names in most computer algebra systems, and in computational mathematics.