4
Example
•Recall: EVEN(A) is not expressible in FO
•Let s<,+ = {<,+}
•Consider A 2 STRUCT[s<,+] s.t.
–< is interpreted as a linear order a1 < … < an
–(ai, aj, ak) 2 +     iff      i+j=k
•Then, we can write a sentence f that checks EVEN(A)   [  HOW ?? ]