Heat is a good suggestion.
Also, if it is a through bolt and you can access the other end of it.
you can drill a hole in the unbroken end, and if the drill grabs it
will spin it out the same as a left hand bolt would.
I recently sheared the head off of a seat clamp bolt and was able
to get a vise-grip on the free end and rotate it to screw it all the
way through and out the other end.
This message thread led me to finally buy a set of left hand drill bits
on Ebay too. Haven’t needed them yet. That’s the problem - you have
to buy them when you don’t actually need them.
You cab get broken extractors out by breaking them to pieces.
A spring loaded center punch sometimes helps - or an air chisel
with a pointy (prick punch) bit.