What is the formula?

Question

// sequence of number
1,3,7,15,31,63,127...
Write a program with recursive function.
When user enter 6 display 63.

Answer 1

Whar you tried so far..?
Did you find logic..?

Answer 2

u[1] = 1
u[i] = u[i-1]*2 + 1

so:

u[2] = 1*2 + 1 => 3
u[3] = 3*2 + 1 => 7
u[4] = 7*2 + 1 => 15
u[5] = 15*2 + 1 => 31
u[6] = 31*2 + 1 => 63
...

or, in a non recursive way:

u[i] = 2^i - 1

(where ^ is the power operator)

Answer 3

Ok~thank you very much