LaTeX.org

Is there any way to use \genfrac or any other command to write three vertically aligned symbol in the following way?

xxxxxxxxxxxxxxxyyyyyyyyyyyyyyyyyyy
s
t
Is there any way to use something like \command{y}{s}{t} ?

I'm writing my PhD thesis.I'm facing some problem relating to incorporating definitions,theorems and examples numbering per chapter.
What I'm looking for is that
first definition in chapter 1 should be numbered as 1.1 and in chapter 2 it should be numbered as 2.1 and so on and same for theorems etc ...

I'm writing my PhD thesis in LaTex. I'm facing a major problem of a lot of spaces between figures and its caption. Please let me know how to reduce spaces between figures and its caption.