a) Name the first 10 natural numbers that have an odd number of divisors (including one and the number itself).
b) Try to formulate and prove a rule that allows finding the next such numbers.
b) As shown in problem $\underline{30365}$, a natural number has an odd number of divisors if and only if it is a perfect square.

## Answer

a) $1,4,9,16,25,36,49,64,81,100$.