Yate’s Algorithm for the Zeta Transform

This is my first post in the folklore knowledge category about an algorithm first used by Yate. I stumbled over this technique while reading the beautiful paper “Determinant Sums for Undirected Hamiltonicity” by A. Björklund (more on this in a few days). Since I did not found an easy to understand presentation of this technique, I decided to write my own one.

When designing algorithms one is sometimes faced with the problem of computing the so-called Zeta Transform \hat{f} of a function f:2^N\rightarrow R, i.e. f maps subsets of a given superset N onto elements in a fixed computation ring R. The Zeta Transform is defined as

\hat{f}(Y):=\sum_{X\subset Y}f(X)\hspace{1cm}\forall Y\subset N\enspace.

Hence, the Zeta Transform is an operator mapping a function f:2^N\rightarrow R onto another function \hat{f}:2^N\rightarrow R. The naive way of computing \hat{f} for all Y\subset N would be to simply compute \hat{f}(Y) from scratch for every Y. In terms of ring additions, this yields a complexity of

\sum_{Y\subset N}2^{|Y|}=\mathcal{O}(3^{|N|})

assuming that the evaluation of the function f itself can be done in unit time.

Yeta’s algorithm gives a more efficient way to do so. Clearly, the naive computation contains a lot of double calculations which should be avoided. Essentially the trick is to derive a simple recursion formula that allows to apply dynamic programming. This allows to prove the following result.

Theorem (Yate’s Fast Zeta Transform). For every function f:2^N\rightarrow R, the Zeta Transform \hat{f}(Y) can be computed for all Y\subset N with \mathcal{O}(N2^N) ring additions.

Proof. For convenience we write N=\{1,\ldots,n\}. For every Y\subset N and i=0,\ldots,n we define

g_i(Y):=\sum_{Y\setminus\{1,\ldots,i\}\subset X\subset Y}f(Y)

describing partial sums over all subsets X\subset Y covering all elements in Y\setminus\{1,\ldots,i\}. Obviously we obtain g_0(Y)=f(Y) for every Y and g_n(Y)=\hat{f}(Y). Now the important observation is the following recursion formula for g_i(Y) given by

g_i(Y)=\begin{cases}g_{i-1}(Y)&\text{ for} i\notin Y\\g_{i-1}(Y)+g_{i-1}(Y\setminus\{i\})&\text{ else}\end{cases}\enspace.

Clearly, for i\notin Y it holds g_i(Y)=g_{i-1}(Y). For the other case, observe that every subset X\subset Y that occurs in g_i(Y) either contains i or not and covers all of Y\setminus\{1,\ldots,i-1\} which explains the second equation in the above recursion formula. Now, the following dynamic programming algorithm allows to finally prove the theorem.

Input: f,N
Output: \hat{f}(Y) for all Y\subset N

  • For all Y\subset N set g_0(Y)=f(Y)
  • For all i=1,\ldots n do
    • For all Y\subset N update g_i(Y) using the above recursion formula
  • Output g_n(Y) for all Y\subset N.

The first step takes 2^N evaluations of f and the second step yields a total number of N2^N ring additions.


About theo rem

I'm a 28 year old PhD student at the Ruhr-University of Bochum. I'm doing my studies in the field of mathmatics with focus on cryptography. I'm particularly interested in cryptanalysis and algorithmics.
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