|   | 1 |  initial version  | 
Does this solve your problem?:
R.<x>=IntegerModRing(15)[]
f=1+x^2+4*x^6
coeffs=(f.coefficients(sparse=False))
show(coeffs)
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.