MathDB

The number

1

is special. The number

2

is special because it is relatively prime to

1

. The number

3

is not special because it is not relatively prime to the sum of the special numbers less than it,

1 + 2

. The number

4

is special because it is relatively prime to the sum of the special numbers less than it. So, a number bigger than

1

is special only if it is relatively prime to the sum of the special numbers less than it. Find the twentieth special number.

Problem Statement