“…Important instruments in those investigations are classical linear topological invariants (approximative and diametrical dimensions, see, e.g., [2,16,20,24]). These invariants are used at their best for regular Köthe spaces [10,11,13,14,17,25,27], in particular, for the spaces (1) with a i ↑ ∞. Problem 1 for nonMontel (a i → ∞) spaces (1), which turns to be beyond powers of the classical invariants, was investigated in [21][22][23] (with l 2 -norms instead of l 1 -norms) by means of some new invariants based on spectral behavior of the operator generating the scale; these invariants exerted an influence on further development of linear topological invariants dealing with non-regular spaces, especially on its early stage).…”