The hidden subgroup problem for ℤ for infinite-index subgroups
Shor's algorithm, later generalized by Kitaev, is an algorithm to solve the hidden subgroup problem in ℤK in quantum polynomial time, uniformly in all parameter, when the hidden subgroup H has finite index. Explicitly, that algorithm takes as functional input a function f: ℤK → S, where S is an unstructured set. The function f has the property that f(x) = f(y) if and only if x-y ∈ H for a hidden subgroup H, and it must itself have a fast algorithm; the Shor-Kitaev algorithm then uses f to calculate H. It is sometimes stated explicitly that H is a finite-index subgroup, or equivalently has maximal rank k; and sometimes only implicit from the condition that S is a finite set. I will present a generalized algorithm that can find H in quantum polynomial time even when H has infinite index or some lower rank. The generalization requires a clear picture of the statistics of the quantum Fourier measurement in this case, and then use of the LLL algorithm to recover H from Fourier samples