[1] **viXra:1303.0172 [pdf]**
*submitted on 2013-03-22 20:54:42*

**Authors:** Ton Kloks, Yue-Li Wang

**Comments:** 14 Pages.

Let G and H be two cographs.
We show that the
problem to determine whether H is a retract of G
is NP-complete.
We show that this problem is fixed-parameter tractable when
parameterized by the size of H.
When restricted to the class of threshold graphs or
to the class of
trivially perfect graphs, the problem becomes tractable
in polynomial time. The problem is also
solvable in linear time
when one cograph is given as an induced subgraph
of the other. We characterize absolute retracts
for the class of cographs. Foldings generalize retractions.
We show that the problem to fold a
trivially perfect graph onto a largest possible clique is
NP-complete. For a threshold graph this folding number equals
its chromatic number and achromatic number.

**Category:** Combinatorics and Graph Theory