IBM Research | Ponder This | October 2012 solutions
Skip to main content

October 2012

<<September October November>>


The cubes can be seen as polynomials, where tossing a cube gives n with a probability proportional to the coefficient of the monomial x^n.

Using this notation, it is easy to see that summing several cubes is the same as multiplying their polynomials.

So all we have to do is take the seven polynomials:

x+x^2+x^3+x^4
x+x^2+x^3+x^4+x^5+x^6
x+x^2+x^3+x^4+x^5+x^6+x^7+x^8
1+x+x^2+x^3+x^4+x^5+x^6+x^7+x^8+X^9
1+x+x^2+x^3+x^4+x^5+x^6+x^7+x^8+X^9
x+x^2+x^3+x^4+x^5+x^6+x^7+x^8+X^9+X^10+x^11+X^12
x+x^2+x^3+x^4+x^5+x^6+x^7+x^8+X^9+X^10+x^11+X^12+...+X^20

Factor them and then multiply the factors together in a different way.

There are many different ways to do this.

A possible solution, in the format we asked for, is

0 5 5 5 5 10 10 10 10 10 10 10 10 5 5 5 5
3 3 6 6 6 9 9 12 12 12 9 9 6 6 6 3 3
3 9 12 12 12 9 3 0 0 0 3 9 12 12 12 9 3
1 2 4 6 8 10 11 12 12 12 11 10 8 6 4 2 1


If you have any problems you think we might enjoy, please send them in. All replies should be sent to: webmster@us.ibm.com