| Dedicated to Professor Constantin Fetecau on his 80th anniversary
Gheorghe Moroșanu* Read full article *gheorghe.morosanu@ubbcluj.ro, Babeș-Bolyai University, Cluj-Napoca, Romania & Academy of Romanian Scientists, Bucharest |
PUBLISHED in Annals Academy of Romanian Scientists Series on Mathematics and Its Application, ISSN ONLINE 2066 – 6594 |

