Let $X_1, ..., X_m$ be a set of $m$ statistically dependent sources over the
common alphabet $\mathbb{F}_q$, that are linearly independent when considered
as functions over the sample space. We consider a distributed function
computation setting in which the receiver is interested in the lossless
computation of the elements of an $s$-dimensional subspace $W$ spanned by the
elements of the row vector $[X_1, \ldots, X_m]\Gamma$ in which the $(m \times
s)$ matrix $\Gamma$ has rank $s$. A sequence of three increasingly refined
approaches is presented, all based on linear encoders.
The first approach uses a common matrix to encode all the sources and a
Korner-Marton like receiver to directly compute $W$. The second improves upon
the first by showing that it is often more efficient to compute a carefully
chosen superspace $U$ of $W$. The superspace is identified by showing that the
joint distribution of the $\{X_i\}$ induces a unique decomposition of the set
of all linear combinations of the $\{X_i\}$, into a chain of subspaces
identified by a normalized measure of entropy. This subspace chain also
suggests a third approach, one that employs nested codes. For any joint
distribution of the $\{X_i\}$ and any $W$, the sum-rate of the nested code
approach is no larger than that under the Slepian-Wolf (SW) approach. Under the
SW approach, $W$ is computed by first recovering each of the $\{X_i\}$. For a
large class of joint distributions and subspaces $W$, the nested code approach
is shown to improve upon SW. Additionally, a class of source distributions and
subspaces are identified, for which the nested-code approach is sum-rate
optimal.Comment: To appear in IEEE Journal of Selected Areas in Communications
(In-Network Computation: Exploring the Fundamental Limits), April 201