I agree with the above suggestions to read the section on Sheldon Brown's site regarding stuck seatposts.........I had just such a problem recently, coincidentally, with an old Centurion, too. What I do to solve the problem, and it's always worked for me, is this: first use very lightweight penetrating oil in the seat tube, both from the top and bottom sides if possible. Let soak at least overnight. Then take an old, junker saddle that you care absolutely nothing about and install it on the seatpost. Take a hammer, and at first with moderate force, begin hitting the forward portion of the junker saddle. If you feel the seatpost move, problem solved, just move to the other side, hit it another time or two to move it a little more, and at this point you should be able to straddle the bike and remove the post by hand. If it doesn't move after the first series of moderate blows, increase the force you hit it with, moving from one side to the other after every few blows. If it's stuck so badly that it won't budge using this method, it's probably time to use some of the more extreme measures listed on Sheldon's site. I will say that I've had to beat the crap out of an old junker saddle to get it done, but I've been fortunate in that I've always been able to free a stuck post using this method. There are worse cases than what I've dealt with, I'm sure, but the good thing about this method is that if it works, the only thing you've damaged is the old junker saddle, because you're striking it forward of the clamp area on the post. Good luck-