# a problem about binomial coefficients sum

I want to know how to caculate $\binom{2n}{0}^3-\binom{2n}{1}^3+\cdots+(-1)^k\binom{2n}{k}^3 \cdots+\binom{2n}{2n}^3$? The sum equals $(-1)^{n}\binom{3n}{2n}\binom{2n}{n}$, but I donot know how to get this.

thanks

• I was about to help with your latest question on irreducibles, but could not because it was deleted. This is not nice to all those who spent valuable time thinking about it. Apr 9, 2012 at 17:03
• Why not reopen it, and rephrase the question as follows: can an irreducible element become reducible in a localization? This uses the standard definition of irreducible, so avoids the distracting comments about units and nonstandard definitions. This is a good question, so deserves to answered. Apr 9, 2012 at 17:14
• @BillDubuque, thank you. As someone said that this is an easy exercise, so I try some example. e.g. $A=\mathbb{Z}[\sqrt{-5}]$, then $2$ is irreducible in $A$, but $2$ is reducible in $A_3$.
– wxu
Apr 9, 2012 at 17:18
• @BillDubuque, I donot know how to reopen it. I have voted to delete it. Then it disappeared..
– wxu
Apr 9, 2012 at 17:20
• In such cases it is better to answer your own question. That way you can get feedback on your answer, and may learn other answers too. Apr 9, 2012 at 17:35