Coefficients of a polynomial defined by its roots : pcoeff pcoef

### 2.24.20 Coefficients of a polynomial defined by its roots : `pcoeff pcoef`

`pcoeff` (or `pcoef`) takes as argument a list of the roots of a polynomial P.
`pcoeff` (or `pcoef`) returns a univariate polynomial having these roots, represented as the list of it’s coefficients by decreasing order.
Input :

`pcoef([1,2,0,0,3])`

Output :

`[1,-6,11,-6,0,0]`

i.e. (x−1)(x−2)(x2)(x−3)=x5−6x4+11x3−6x2.