x^16 + 4 x^15 + 10 x^14 + 20 x^13 + 35 x^12 + 52 x^11 + 68 x^10 + 80 x^9 + 85 x^8 + 80 x^7 + 68 x^6 + 52 x^5 + 35 x^4 + 20 x^3 + 10 x^2 + 4 x + 1 .
See Melody's answer to this question to learn how to calculate the coefficients:
http://web2.0calc.com/questions/what-is-the-coefficient-of-a-4b-2-in-the-expansion
