What do empty white circles mean in a quantum circuit?

Question

I'm studying Shor's algorithm. but I see from the beginning this empty circle. what this circle means??

Answer 1

It is a "negative" control.

In other terms, the gate in your circuit will apply a NOT on the last qubit if and only if the following conditions are both met:

• $\vert x_3\rangle = \vert 0 \rangle$
• the first ancilla qubit is in the state $\vert 1 \rangle$.

A "normal" control (i.e. not an empty circle) would change the first condition to $\vert x_3 \rangle = \vert 1 \rangle$.