Processing math: 100%

Thursday, November 12, 2015

Probabiliy: Who has the winning strategy?

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)=x2012?

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:

2011k=0(ak2012k)+20122012=0

or:

2011k=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 (a where 02011) which can be obtained from:

1k=0(ak2012k)+a2012+2011k=+1(ak2012k)=20122012

Solving for a, we find:

a=12012(1k=0(ak2012k)+2011k=+1(ak2012k)+20122012)

With this value for a, Jessica is assured of winning the game.

2 comments:

  1. Beautiful explanation on this problem Isabelle. Well done my friend.(Y)

    ReplyDelete
  2. This beautiful explanation belongs to Mark! :P

    ReplyDelete