Generalised Free Products which are Free Products of Locally Extended Residually Finite Groups. R.B.J.T. Allenby; R.J. Gregorac — 1971 Mathematische Zeitschrift
On Baica's trigonometric identity. Gregorac, R.J. — 1989 International Journal of Mathematics and Mathematical Sciences
On polynomials with simple trigonometric formulas. Gregorac, R.J. — 2004 International Journal of Mathematics and Mathematical Sciences