Consider a polynomial
P(x)=a0+a1x+⋯+a2011x2011+x2012
Nigel and Jessica are playing the following game. In turn, they choose one of the coefficients a0,⋯,a2011 and assign a real value to it. Nigel has the first move. Once a value is assigned to a coefficient, it cannot be changed any more. The game ends after all the coefficients have been assigned values.
Jessica's goal is to make P(x) divisible by a fixed polynomial m(x) and Nigel's goal is to prevent this.
Which of the players has a winning strategy if m(x)=x−2012?
Solution provided by Mark, another contributor from this blog:
In order to win, Jessica wants to have at the end (since she has the last move):
P(2012)=0
or:
2011∑k=0(ak2012k)+20122012=0
or:
2011∑k=0(ak2012k)=−20122012
No matter what values Nigel and Jessica have chosen for the first 2,011 coefficients, all Jessica has to do is choose for the last (aℓ where 0≤ℓ≤2011) which can be obtained from:
ℓ−1∑k=0(ak2012k)+aℓ2012ℓ+2011∑k=ℓ+1(ak2012k)=−20122012
Solving for aℓ, we find:
aℓ=−12012ℓ(ℓ−1∑k=0(ak2012k)+2011∑k=ℓ+1(ak2012k)+20122012)
With this value for aℓ, Jessica is assured of winning the game.
Beautiful explanation on this problem Isabelle. Well done my friend.(Y)
ReplyDeleteThis beautiful explanation belongs to Mark! :P
ReplyDelete