I am writing the following:

$$p(Z_{n+m}=i for some m> 0|Z_{n}=i)=1$$ 

Compiling, the spaces between the words "i for some m" are removed giving as a result "iforsomem"

How can I prevent this?

Note that I cannot write something like

$p(Z_{n+m}=i$ for some $m> 0|Z_{n}=i)=1$ 

because I want it centered horizontally

Thank you for your help!