Question 220340
Simply graph {{{x^2 + y^2 = 4}}} (note: solve for 'y') to get


{{{ drawing(500, 500, -10, 10, -10, 10,
 graph( 500, 500, -10, 10, -10, 10,0),
 circle(0,0,2)

)}}}



Since the graph does NOT pass the vertical line test, it is NOT a function.