The function is defined recursively. In order to calulate f(3), f(2) and f(1) must be known. f(1) is given
f(1) = 2
f(2) is then calculated
f(2) = 2 * f(1) + 1 = 2 * 1 + 1 = 2 + 1 = 3
Finally f(3) can be calculated given f(2)=3
f(3)= 2 * f(2) + 1 = 2 * 3 + 1 = 6 + 1 = 7
