What is the smallest positive integer n which CANNOT be written in any of the following forms ?
• n = 1 + 2 + · · · + k for a positive integer k.
• n = p^k , for a prime number p and integer k.
• n = p + 1 for a prime number p
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To find the smallest integer n with these properties, it is enough to check that every positive integer up to n − 1 does not satisfy all the conditions.
1 = 2 0 , condition 2
2 = 2 1 , condition 2
3 = 3 1 , condition 2
4 = 3 + 1 , condition 3
5 = 5 1 , condition 2
6 = 5 + 1 , condition 3
7 = 7 1 , condition 2
8 = 2 3 , condition 2
9 = 3 2 , condition 2
1 0 = 1 + 2 + 3 + 4 , condition 1
1 1 = 1 1 1 , condition 2
1 2 = 1 1 + 1 , condition 3
1 3 = 1 3 1 , condition 2
1 4 = 1 3 + 1 , condition 3
1 5 = 1 + 2 + 3 + 4 + 5 , condition 1
1 6 = 2 4 , condition 2
1 7 = 1 7 1 , condition 2
1 8 = 1 7 + 1 , condition 3
1 9 = 1 9 1 , condition 2
2 0 = 1 9 + 1 , condition 3
2 1 = 1 + 2 + 3 + 4 + 5 + 6 , condition 1
2 2 isn't the sum of the first k integers, because when k = 6 the sum is 2 1 and when k = 7 the sum is 2 8 . It's not a power of a prime, either, because it is the product of two distinct primes, and it's not 1 more than a prime, because 2 1 is composite.
The correct answer is 2 2