On First-Order Definable Colorings
  We address the problem of characterizing $H$-coloring problems that are first-order definable on a fixed class of relational structures. In this context, we give several characterizations of a homomorphism dualities arising in a class of structure. 
Jaroslav Nesetril (IUUK), Patrice Ossona De Mendez (IUUK, CAMS)
math.CO