1. The first student switched on all lights.

2. The second one switches off all switches that are multiples of two.

3. The third one switched off all on lights and switched on the off lights that were multiples of 3.This continued for 4, 5, 6 and so on.

If the professor had 1000 bulbs and each of the 1000 students did as told, find the number of bulbs that remained on after the exercise.