I am replacing a kitchen sink strainer. A large nut at the top of the drain pipe is frozen and I cannot remove it. It has been suggested that remove the pipe entirely and replace with pvc pipi. Problem is the pipe seems frozen and I am unable to loosen it. May it have been cemented and if so what can you suggest?

That is most likely a polypropylene tailpiece. The nut is frozen not cemented. Spray some WD40 on the nut at the strainer, wait a while and try to remove. Remove counter clockwise.

For that stuff, simply take a hacksaw and cut the nut. Is a disposal involved? Let me know. Back to you, Tom