What is the Fibonacci sequence number?

Answer 1

It is a sequence defined by the following rule:

U(1) = 1

U(2) = 1

U(n) = U(n-2) + U(n-1) for n = 3, 4, 5, ...

In other words, the first two numbers in the sequence are both 1. After that, each number is the sum of the two numbers before it.

Some versions start with U(1) = 0. The only effect of this is that the position of each number in the sequence is increased by 1.