4, 9, 25 and 49
They are squares of prime numbers.
1 has one factor.
The numbers for which the sum of its proper factors is 1 less than the number is the powers of 2; less than 50 these are: 2, 4, 8, 16 & 32.
64.
30
Every prime number has exactly two factors. They are '1' and the number itself. There are 15 prime numbers less than 50. A few of them are 2, 3, 5, 7, 11, 13, and 47.
48 has 10 factors, the most factors of any number under 50.
48
42
1 has one factor.
30, 40, 42
The numbers for which the sum of its proper factors is 1 less than the number is the powers of 2; less than 50 these are: 2, 4, 8, 16 & 32.
39
64.
36 The factors of 36 are : 36,18,12,9,6,4,3,2 and 1.
30
It is 30.
I think it's 51.