Hello guys!!I am from Greece so sorry for my bad english...
I have a problem at propositional logic...specifically at the proofing system...

the excercise which made me crazy is the following...
proove : \vdash ((\phi \rightarrow{\psi})\rightarrow{\neg{\chi}})\rightar  row{(\chi} \rightarrow{\neg{(\phi \rightarrow{\psi})}})<br />
we can use the following axioms and theorems...

\vdash ((\phi \rightarrow{(\psi \rightarrow{\sigma})})\rightarrow{((\phi \rightarrow{\psi})\rightarrow{(\phi \rightarrow \sigma})})


\vdash \phi \rightarrow{(\psi \rightarrow{\phi})}

\vdash(\neg{\phi} \rightarrow{\neg \psi} )\rightarrow{((\neg \phi \rightarrow{\psi})\rightarrow{\phi})}

 \phi \rightarrow \psi  \vdash \neg{\psi} \rightarrow \neg{\phi}

 \vdash \phi \rightarrow \psi   \equiv \phi \vdash \psi


thanx for advance for your help...

if u know any good source to study about axiomatic system plz tell me thanx again...