1983 · R. D. Ekers, Jh Vangorkom, et al. · University of Groningen research database (University of Groningen / Centre for Information Technology) · 44
1983 · G. S. Shostak, Jh Vangorkom, et al. · University of Groningen research database (University of Groningen / Centre for Information Technology) · 1
1985 · P. A. Shaver, No Value Pottasch, et al. · University of Groningen research database (University of Groningen / Centre for Information Technology)