Build a polynomial from it’s evaluation : genpoly

### 2.27.3 Build a polynomial from it’s evaluation : `genpoly`

`genpoly` takes three arguments : a polynomial P with n−1 variables, an integer b and the name of a variable `var`.
`genpoly` returns the polynomial Q with n variables (the P variables and the variable `var` given as second argument), such that :

- `subst(Q,var=b)==P`
- the coefficients of Q belongs to the interval ]−b/2 ; b/2]

In other words, P is written in base b but using the convention that the euclidean remainder belongs to ]−b/2 ; b/2] (this convention is also known as s-mod representation). Input :

`genpoly(61,6,x)`

Output :

`2*x```^```2-2*x+1`

Indeed 61 divided by 6 is 10, remains 1, then 10 divided by 6 is 2 remains -2 (instead of the usual quotient 1 and remainder 4 out of bounds),

61=2*62−2*6+1

Input :

`genpoly(5,6,x)`

Output :

`x-1`

Indeed : 5=6−1
Input :

`genpoly(7,6,x)`

Output :

`x+1`

Indeed : 7=6+1
Input :

`genpoly(7*y+5,6,x)`

Output :

`x*y+x+y-1`

Indeed : x*y+x+y−1=y(x+1)+(x−1)
Input :

`genpoly(7*y+5*z```^2```,6,x)`

Output :

`x*y+x*z+y-z`

Indeed : x*y+x*z+y−z=y*(x+1)+z*(x−1)