 
Summary: A nontopological proof for
the impossibility of kset agreement
Hagit Attiya1
and Armando Casta~neda2
1
Department of Computer Science, Technion, Haifa 32000, Israel.
hagit@cs.technion.ac.il
2
IRISAINRIA, Campus de Beaulieu, 35042 Rennes Cedex, France.
armando.castaneda@inria.fr
Abstract. In the kset agreement task each process proposes a value,
and it is required that each correct process has to decide a value which
was proposed and at most k distinct values must be decided. Using topo
logical arguments it has been proved that kset agreement is unsolvable
in the asynchronous waitfree read/write shared memory model, when
k < n, the number of processes.
This paper presents a simple, nontopological impossibility proof of kset
agreement. The proof depends on two simple properties of the immediate
snapshot executions, a subset of all possible executions, and on the well
known graph theory result stating that every graph has an even number
