The notation choice in chapter 2 is tricky, because you're mixing mathematical and programming function notation. In DDD terms, set theory and the Java programming language are two different bounded contexts. This is tricky because there might not be a good solution, but it’s worth mentioning and trying to be consistent.
Perhaps this is worth a sidebar or a table to compare and separate mathematical and Java notation for functions: function names, parameters, types, inverse, etc.
For example, section 2.1.1.2 (Inverse functions) mentions ‘the set of signed integers (positive and negative, noted as Z)’. It would be helpful to be clear that this is mathematical language, and to use the right symbol ('fat' Z with a double stroke), and perhaps to note that 'the set of integers' (mathematics) corresponds to the concept of the Java Integer type.
Similarly, section 2.1.1.5 (Functions of several arguments) mentions 'N x N'. It would be helpful to mention that this is the cross product of two sets, and say what this means (handle or declare). Also, the lowercase 'x' should be a multiplication sign '×'.
