1.Heat as in Oxy torch, MAPP gas is one way to go. Alternative to a Stilson is to drill a suitable dia hole through the tube and use a length of steel rod as a tommy bar. The you can twist and pull.
Did I say heat and lots of it is the way to go? Mind you, it ruins the paint work.
2.An alternative is to use chemistry and dissolve the seat post. I have done this successfully on an a tandem. You will need to plug the tube below the seat post, ie at the bottom bracket end.
3. If you have cut off the top of the tube, you could possibly saw an axial slit and then try and squeeze the tube to a smaller diameter. It should then withdraw easily.
4. Did I say that heat is the way to go? See 1. above.
If you want to dissolve it PM me and I will give the link.
I have a personal interest in your outcome because I have another stuck seat post in a tandem.
Stay fixed. Keeping a cool head.