21. Let $p$ be a prime number such that the next larger number is a perfect square. Find the sum of all such prime numbers. (For example, if you think that 11 and 13 are two such prime numbers, then the sum is 24 .)
21 Ans: 3 .
If $p+1=n^{2}, p=(n+1)(n-1)$. Since $p$ is prime, $n=2$. Hence $p=3$ is unique.