then the system has also two behavoiurs one for y < 0 and another one for y > 15 .
Unfortunately each time as the system reaches y = 15, its switching to the second exponential behaviour, but then the solutions also decreases so you arrive again at y < 0, and you switch again,
... which gives a neverendind story, and of course the computation never converges !
In order to obtain a behaviour until y reaches 15 and another one after (even if y < 15 again !), you have to introduce an internal parameter, governing the swich: alpha = 0 until y = 15 alpha = 1 if y was already been 15 ... which means another equation to your problem of the sort:
alpha == If[ alpha == 1 || y == 15 , 1 , 0 ]
a + andrei
______________________________________________________________________ Andrei Constantinescu firstname.lastname@example.org