Generating polynomials for a multistep method

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dynamicskillingme
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Hi, I'm struggling to understand how the generating polynomials work and are implemented in the difference equation for a general ODE y' = f(t,y)
Difference Equation
D%20h%20%5Csum_%7Bj%3D0%7D%5E%7Bk%7D%20b_%7Bj%7D%20f%28t_%7Bn+j%7D%2C%20y_%7Bn+j%7D%29.gif

Generating polynomials
%7D%20%5C%5C%20%5Csigma%20%28w%29%20%3D%20%5Csum%5E%7Bk%7D_%7Bj%3D0%7D%20b_%7Bj%7D%20w%5E%7Bj%7D.gif

"Coefficients are normalized either by a_k = 1 or sigma(1) = 1
 
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Sorry if I wasn't clear enough, I don't understand the concept behind the generating polynomials. My notes state the examples of the three theta methods but I can't understand how they are obtained (e.g. implicit euler is sigma(w) = w )