... Math., 38 (20 05), No .2, 36 5 -38 6. [33 ] Nguyễn Thành Long, Lê Văn Út, Nguyễn Thò Thảo Trúc, On a shock problem involving a linear viscoelastic bar , Nonlinear Anal., 32 (20 05), 198 -22 4. [34 ] ... Math. 13 (1994), 22 5- 23 3 . [35 ] L.A. Medeiros, J. Limaco, S.B. Menezes, Vibrations of elastic strings : Mathematical aspects, Part one , J. Computational Anal. Appl., 4, No. 2, (20 02) , 91- 127 . ... conditions , J. Math. Anal. Appl. 27 4, No.1 (20 02) , 1 02- 1 23 . [28 ] Nguyễn Thành Long, On the nonlinear wave equation ,,( 2 utBu tt − ),,,,,,,(), 22 222 txtxxxtx uuuuuutxfuuu = associated with the...