What is the supply curve of a firm in the short run?
The short run supply curve of a firm shows the levels of output that the firm chooses to produce corresponding to different values of the market price, keeping technology and prices of factors of production unchanged.
When the market price is greater than or equal to the minimum AVC, the firm produces a positive output level where the price equals SMC on the rising part of the SMC curve. However, when the market price is less than the minimum AVC, the firm produces zero output as it cannot cover its variable costs.
Thus, the short run supply curve of a firm is the rising part of the SMC curve from and above the minimum AVC together with zero output for all prices strictly less than the minimum AVC.
Explanation
The textbook clearly defines the short run supply curve in section 4.4.1 by combining two cases. Case 1 establishes that when price minimum AVC, the firm supplies positive output where P = SMC (on the rising part). Case 2 establishes that when price < minimum AVC, the firm supplies zero output. The supply curve is essentially the portion of the SMC curve that lies above the AVC curve, as the firm will not produce at prices below minimum AVC.