[1] J. Havrda: 
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