1. 1University Politehnica Timişoara, Department of Mathematics Timişoara, 300006, Romania
2. 2University Politehnica Timişoara, Department of Mechanics and Vibration, Timişoara, 300222, Romania and Department of Electromechanics and Vibration, Center for Advanced and Fundamental Technical Research, Romania Academy, Timişoara, 300223, Romania
3. 3University Politehnica Timişoara, Department of Applied Electronics, Timişoara, 300223, Romania