Get help from a friend to heat the frame evenly over the area the steel tube is in while you try to work the steel post back and forth. Heating the aluminum should make it expand slightly before the heat transfers to the steel ,giving you a little wiggle room to get it to break loose.

That's cool! ........... Sorry I could not resist that...LOL

I admit I have not tried heat. I believe the shaft is about 10 inches in with 6 inches sticking out. I have had aluminum /alloy wheels that corroded to the steel brake hubs....Steel aand aluminum do not go well together. I know with service entrance aluminum cables we had to use "NO-LOX" on the cables before lugging to the panels.

Try heating the Steel seat post near the Alum. Frame moving the propane torch back & forth -up & down from one metal to the other -The dissimilar metals will Expand/Contract at different times -tapping gently with a hammer will help as you move the Torch around continuously! This is where Anti Seize helps Before the Problem starts!

a can of compressed “air” from an office supply store sprayed upside down will give you a nice -60F liquid that will rapidly cool anything it is sprayed on.

Pharmacy also sells small cans of freez spray for freezing warts. Heat and cold are your friends (kroil too)

after a few drinks the flies appeared to have learned more evasive maneuvers and harder to hit..."Even flies fear the sting of death".

Got on the front brakes yesterday! Interesting find! Crappy mess as i expected! Completely down to metal! The left side was worse, inside shoe was completely gone! Remnants of shoe inside piston. Never seen one this bad , i just ground it back down with the grinder reinstalled and working perfectly, see the remnants of the shoe i dug out of the piston laying on the tire in the background. Had to reinstall it with a c clamp my caliper tool was to large to fit in the tiny caliper.I have new calipers ordered for both sides, but its working good now i suspect the bad one will start leaking. Caliperss are so cheap 25 for set of 2 new caliper is cheaper than piston and seals. I have one more of these Eigers to do, i have the front brakes working with no issues but never pulled the caliper to check it out, rears not working at all, while its fresh on my mind i think i will do it also, plus thats the one i need to mount the new tires on. That will be an interesting venture, i usually take it and have them mounted but the cost has gotten so expensive i decided to do it myself. I bought one of those Harbour Freight portable tire changers cheaper than the cost of getting them mounted, very interested to see how that goes, but that will be another thread.

Yeah I was going to suggest hitting the top of the bolt.. But Gw has covered it well.. And like he says, if you can get it to move, even the smallest amount of rotation, only five degrees even, then tighten it down that five degrees and then try undoing it by five degrees again, keep working that five degrees till if feels a bit freer then try for eight degrees, work that eight degrees.. Keep doing that, a lot of small gains. Even if it takes an hour to get it free before you can start winding it up, it will be better than breaking the bolt off. And when you are winding it out, keep winding it back down again by at least a half turn every full turn or the bolt will cause galling and wreck the threads.. Oh.. and only wind it slowly by hand. Power tools will wind it too fast and can cause galling even on good threads. Impact tools should only be used to loosen bolts, not wind them off.