simple filter question

[ionone]

Hi

i can't understand what computations does Octave with this simple filter :
b = [1 2 3 4]
aa = [0.1 0.2]
A = filter(1, aa, b)

the result in Octave is : A = 10  0  30 -20

i thought Octave did this : 
                                               
A(1) = 1/0.1 (this number is correct)
A(2) = 2/0.1 + 1/0.2 = 25 (differs from 0)
A(3) = 3/0.1 + 2/0.2 = 40 (differs from 30)
A(4) = 4/0.1 + 3/0.2 = 55 (differs from -20)
...
please help me understand what computations does Octave in this case
I've turned the equations in every possible way without getting it :/

many thanks.

Jeff



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