2. The double factorial of a positive integer represents all positive integer products that do not exceed this positive integer and have the same parity with it. The double factorials of the first 10 natural numbers (from 0 to 9) are 1, 1, 2,3,8,15,48,105,384,945 respectively.

3, the working principle of the motor: (omitted. Is it okay? )

4. Working principle of oil pump: There are many kinds of oil pumps, mainly piston pump, vane pump, gear pump, screw pump, etc. The working principles are different (not to mention one by one), but they all change oil without pressure into oil with pressure.