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Independence in a set with oгthogonality. Časopis pěst. mat. 107 (1982), 267-272.
MR 0673052
[2] J. Havrda:
Pгojection and covering in a set with oгthogonality. Časopis pěst. mat. 112 (1987), 245-248.
MR 0905970
[3] J. Havrda:
A study of independence in a set with oгthogonality. Časopis pӗst. mat. 112 (1987), 249-256.
MR 0905971