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3. J. Vanderschueren, doctoral thesis, Université de Liège, Belgium (1974).
4. J. Vanderschueren and J. Gasiot, “Thermally Stimulated Relaxation in Solids”, Topics in Applied Physics, Vol. 37, edited by P. Braünlich (Springer, Berlin, 1979).
5. J. Belana, doctoral thesis, Universidad de Barcelona, Spain (1978).