[1] A. V. ARCHANGEL'SKII : Классы топологических групп. Успехи Матем. Наук 36 (1981), 127-146.
[3] J. G. CEDER:
Some generalisations of metric spaces. Pacific J. Math. 11 (1961), 105-126.
MR 0131860
[5] R. W. HEATH R. E. HODEL:
Characterizationa of $\sigma $-spaces. Fund. Mat. 77 (1973), 271-275.
MR 0321016
[6] A. OKUYAMA:
On metrisability of M-spaces. Proc. Japan Acad. 40 (1964), 176-179.
MR 0167957
[7] A. OKUYAMA:
A surrey of the theory of $\sigma $-spaces. General Topology and Appl. 1 (1971), 57-63.
MR 0287507
[8] P. SIWIEC J. NAGATA:
A note on nets and metrization. Proc. Japan Acad. 44 (1968), 623-627.
MR 0242116
[9] A. P. ŠOSTAK: Some remarke about bijections onto metrizable spaces. to appear in Czech. Math. J.