As nice as it is, integers are not a field (i.e. most of them don't have multiplicative inverses, or 1/n usually isn't integral).
Another good writeup: http://andrea.corbellini.name/2015/05/17/elliptic-curve-cryp...
Another good writeup: http://andrea.corbellini.name/2015/05/17/elliptic-curve-cryp...
Edit: but yes, the OP writes "integers (Z) are a field", which is wrong. Integers form a ring.