Can you start engine without APU?

Can you elaborate on the possibility of starting an engine without an Auxiliary Power Unit (APU)? In aviation, an APU is typically used to provide power to various systems before the main engines are started, including electrical and pneumatic systems. Is there a specific scenario or type of engine where starting it without an APU is feasible or advisable? Are there any potential risks or limitations to consider when attempting to start an engine without an APU? Understanding the context and potential consequences is crucial for ensuring SAFE and efficient operations.