I’ve used the sheldonbrown trick in the past:
If nothing else works, the final resort is the old hacksaw blade trick. Cut the seatpost off so that about 1/2″ is left sticking out, then insert a hacksaw blade into the seatpost and carefully cut a slit in the post. This is very laborious, and you run the risk of damaging the frame if you cut too far, but this approach cannot fail. Once you have cut the slit, grab one edge of the cut with a locking plier and roll the seatpost up inside itself and pull it out.
I cut a groove in a piece of wood to support the hack saw blade used an old inner tube to protect my hand. I also wrapped a bit of wire around the bottom end of the blade that stuck out of the bottom of the post to try to prevent accidentally cutting into the frame when I’d almost cut through it. Took over an hour to do it, but it did work.