In Figure E6-3, which is the schematic symbol for the NOT operation (inversion)?

The NOT operation, also known as an inverter, simply reverses the logic state of its input. If the input is high, the output is low, and vice versa. The schematic symbol for a NOT gate is a triangle pointing in the direction of signal flow, with a small circle (the inversion bubble) at the output apex. In Figure E6-3, symbol 5 illustrates this exact shape. The small circle is the universal symbol for logic inversion in digital schematics, which is why it also appears at the output of NAND and NOR gates.