rcaXB Ris a relation from a to B DemRa。 (a,b)∈R(a,C)∈R (a2b)∈R(a,C)∈ Runless b=c ☆ function DomR=A,(everywhere)function❖ RA×B,R is a relation from A to B， DomRA。 ❖ (a,b)R (a, c)R ❖ (a,b)R (a, c)R unless b=c ❖ function ❖ DomR=A， (everywhere)function