How to change to Mathematics PhD program following M.S. in Computer Science? I wonder if anyone can give me some advice about how a computer science graduate shifts career to mathematics? I'm currently doing a Master's degree in computer science and i was thinking about doing a PhD in pure mathematics? so I was thinking is this transition feasible? how can I make this leap of faith?

Kindly note that currently -after graduation- I nearly don't do mathematics anymore, currently all what I am doing is writing software and administrating open source operating systems. In undergrad levels I took an introductory course in discrete mathematics and some calculus courses.

someone told me: Yes, the transition is feasible but it requires I take some courses and attend some math classes even as a special student (no degrees, no exams). But I thought I can get other opinions and views.
Take a look at Is it easy to change your major after starting your PhD or masters, if it is necessary?, which is somewhat similar. 

In particular, unfortunately, it sounds like your current mathematics background is very far from what would be needed to enter a math PhD program - it's maybe 1/4 of the coursework that would be expected. You would need to have preparation equivalent to that of a BS in mathematics, and that sounds like it would be the equivalent of 2-3 more years of full time coursework for you.

To be honest, I don't think that the coursework you describe can even have given you much of a sense of what pure mathematics is, much less what it is like to do research in the area.  One needs rigorous proof-based classes such as real analysis and abstract algebra.  

I would not really advise you, or anyone, to even think about grad school in mathematics before they have enough of a background to reasonably understand what it involves.