CoefficientList
CoefficientList[poly, var] gives a list of coefficients of powers of var in poly, starting with power 0.
CoefficientList[poly, 
, 
, ... 
] gives an array of coefficients of the 
.
Example: CoefficientList[x^2 + 2 x y - y, 
x, y
]
.
The dimensions of the array returned by CoefficientList are determined by the values of the Exponent[poly, 
].
Terms that do not contain positive integer powers of a particular variable are included in the first element of the list for that variable.
CoefficientList always returns a full rectangular array. Combinations of powers that do not appear in poly give zeros in the array.
CoefficientList works whether or not poly is explicitly given in expanded form.
See The Mathematica Book on the web: Section 3.3.2.
See also: Series, Collect, FactorList.
Further Examples
