(%i1) solve(acot(x),x); cot: argument 0 isn't in the domain of cot. -- an error. To debug this try: debugmode(true); (%i2) solve(x*acot(x),x); cot: argument 0 isn't in the domain of cot. -- an error. To debug this try: debugmode(true);
But x=0 clearly is a solution to the latter, which is defined for all real input.