关于只能使用电脑的原因，请参见我在 math SE meta 上的帖子评论： http://meta.math.stackexchange.com/questions/24978
Firstly, the user-friendly math expression editor that Approach0 is using (i.e. mathquill) does not support mobile phone. Secondly, I think typing math expression on mobile can be very frustrating and inefficient (many symbol input needed). In the future, if we can find some open-source tools that handles hand-writing math input really well, at that time I think we can enable mobile site.