- document
- Gerritsma, J. (author), Beukelman, Wim (author) report 1986
- document
- Beukelman, W. (author), Gerritsma, J. (author) report 1983
- document
- Beukelman, W. (author), Gerritsma, J. (author) report 1982
- document
- Beukelman, W. (author), Gerritsma, J. (author) report 1982
- document
- Gerritsma, J. (author), Beukelman, W. (author) report 1967
- document
- Gerritsma, J. (author), Beukelman, W. (author) report 1965
- document
- Gerritsma, J. (author), Beukelman, W. (author) report 1964