# Singular

#### D.15.13.13 formaldivisorplus

Procedure from library `divisors.lib` (see divisors_lib).

Usage:
formaldivisorplus(A ,B); A + B; A = formaldivisor, B = formaldivisor.

Assume:
A and B are formal divisors on X.

Return:
a formal divisor on X

Theory:
The procedure will add the formal sums.

Example:
 ```LIB "divisors.lib"; ==> Welcome to polymake version ==> Copyright (c) 1997-2015 ==> Ewgenij Gawrilow, Michael Joswig (TU Darmstadt) ==> http://www.polymake.org ring r=31991,(x,y,z),dp; ideal I = y^2*z - x*(x-z)*(x+3*z); qring Q = std(I); divisor A = makeDivisor(ideal(x,z),ideal(1)); divisor B = makeDivisor(ideal(x,y),ideal(1)); divisor C = makeDivisor(ideal(x-z,y),ideal(1)); formaldivisor fE= makeFormalDivisor(list(list(-5,A),list(2,B))); formaldivisor fE2= makeFormalDivisor(list(list(-5,A),list(2,C))); formaldivisorplus(fE,fE2); ==> [1]: ==> [1]: ==> -10 ==> [2]: ==> (z,x) - (1) ==> ==> [2]: ==> [1]: ==> 2 ==> [2]: ==> (y,x) - (1) ==> ==> [3]: ==> [1]: ==> 2 ==> [2]: ==> (y,x-z) - (1) ==> ```