The first sodium-magnesium and sodium-zinc ketimido complexes display contrasting inverse crown ring and pseudo-cubane structures respectively, while a sodium-zinc heterotrianionic alkide-alkoxide-amide adopts a third type of structure with a stepped ladder motif.

In this paper we show that inverting problems of higher complexity is easier than inverting problems of lower complexity. While inverting Σ i 3CNFSAT is known to be coNP-complete [6] for i = 1 we prove that it remains coNP-complete for i = 2 and is in P for all i ≥ 3. Relatedly, we show that inverting Σ i 3DNFSAT is in P for all i ≥ 1.