The sign used for "at least" is "greater than or equal to".
It looks like this : [tex]\ge [/tex]
Here is an example :
[tex]x\quad \in \quad N\quad (x\quad is\quad a\quad Natural\quad number)\\ \\ x\quad is\quad at\quad least\quad 9[/tex]
Can be shown like this : [tex]x\ge 9[/tex].