# Singular

### 4.10.3 map operations

`( )`
composition of maps. If, for example, `f` and `g` are maps, then `f(g)` is a map expression giving the composition of `f` and `g`,
provided the target ring of `g` is the basering of `f`.

map_expression `[` int_expressions `]`
is a map entry (the image of the corresponding variable)

Example:

 ``` ring r=0,(x,y),dp; map f=r,y,x; // the map f permutes the variables f; ==> f[1]=y ==> f[2]=x poly p=x+2y3; f(p); ==> 2x3+y map g=f(f); // the map g defined as f^2 is the identity g; ==> g[1]=x ==> g[2]=y g(p) == p; ==> 1 ```