matlab - Writing sums of product taken two or three at a time -

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i have variables a1 a2 a3 a4.... etc want find out f1 f2 f3 ...fk.... fn where

      f1 =     a1+a2+a3+a4+.....an       f2 = - (sum of products of a's taken 2 @ time )       f3 =   (sum of products of a's taken 3 @ time)       fk =  (-1)^(k+1)* (sum of products of a's taken k @ time)       , on till fn

can me how fk through pseudo code or matlab code. there built-in function in matlab can me in doing ?

use in-built function poly(r) in matlab takes roots of polynomial r array:

function fk = get_comb(r,k)       p=poly(r)    fk=(-1)^(k)*p(k+1); end

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