You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
7 lines
499 B
7 lines
499 B
In order to show that a sequent is not valid, we provide a \emph{counter example}, which is a model that satisfies all premises but falsifies the conclusion.
|
|
|
|
This is a consequence of soundness. We know from the definition of soundness that
|
|
|
|
$$\varphi_1, \varphi_2,\!..., \varphi_n \nmodels \psi \qquad \Rightarrow \qquad \varphi_1, \varphi_2,\!..., \varphi_n \nvdash \psi$$
|
|
|
|
A counterexample is enough to tell us that the left-hand side of this implication is true, hence the sequent is not valid.
|