Hi Peerless...
Assuming we are talking about PCs----
<Alt> 129 = ü
And, that is not the ½ of it!
<Alt> and the number keys on the NUMERIC KEYPAD will provide you with all of the extended character set available.
Have endless hours of fun discovering them, or find the link which gives them to you!
You don't want all the answers too easily, do you?
It is hard enough to remember only some of them!
p.s. <Alt> 137 = ë