## Sequence 1, 3, 7, 12

Which is the next number in the following sequence:

**1, 3, 7, 12, 18, 26, 35, 45, 56, ?**

**SOLUTION**

This is the so called Hofstadter Figure-Figure Sequence.

The sequence of the differences between the consecutive numbers in the original sequence is 2, 4, 5, 6, 8, 9, 10, 11â€¦ These are exactly the natural numbers missing from the original sequence. Therefore, the next number should be 56 + 13 = 69.