Binary decomposition rewrites an integer as the sum of powers of two at the positions of its one-valued binary digits. In quantum function encoding, this turns a key k into the angle multiple kθ needed to construct a geometric sequence state. section 10.1.1
Decomposing k = 4
With n = 3, 4 is written as 100 in binary. The one-valued digit is at position 2, so 4 = 2². For θ = π/4, the same decomposition gives 4θ = 2²θ.
