[1] S. Kurepa: The Cauchy functional equation and scalar product in vector spaces. Glasnik matematicko-fizicki i astronomski 19 (1964), 23-35. MR 0171100 | Zbl 0134.32601
[2] S. Kurepa: Quadratic and sesquilinear functional. Glasnik matematiCko-fizidki i astronomski 20(1965), 79-92. MR 0193390