To try and get some expansion on the seat tube without damaging the paint I've wrapped it in a cloth and then poured on boiling water. You could try some plumber’s pipe freezing spray on the post also.
Drilling a hole through the post put a long strong bolt or bar through and use this to provide additional purchase in a vice.
I would use the caustic soda when all else has failed
To try and get some expansion on the seat tube without damaging the paint I've wrapped it in a cloth and then poured on boiling water. You could try some plumber’s pipe freezing spray on the post also.
Drilling a hole through the post put a long strong bolt or bar through and use this to provide additional purchase in a vice.
I would use the caustic soda when all else has failed