Find the Value of the Determinant

Algebra Level 4

( 2000 0 ) ( 2000 1 ) ( 2000 18 ) ( 2001 0 ) ( 2001 1 ) ( 2001 18 ) ( 2018 0 ) ( 2018 1 ) ( 2018 18 ) \left|\begin{matrix} 2000\choose 0 & 2000\choose 1 & \cdots & 2000 \choose 18 \\ 2001\choose 0 & 2001\choose 1 & \cdots & 2001\choose 18 \\ \vdots & \vdots & \ddots & \vdots \\ 2018 \choose 0 & 2018 \choose 1 & \cdots & 2018 \choose 18 \\ \end{matrix}\right|

Find the value of the 19 × 19 19\times19 determinant above, where ( n k ) n \choose k denotes the binomial coefficient .


The answer is 1.

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1 solution

Mark Hennings
Feb 13, 2018

If M ( n ) M(n) is the n × n n \times n matrix M ( n ) = ( ( 2000 0 ) ( 2000 1 ) ( 2000 n 1 ) ( 2001 0 ) ( 2001 1 ) ( 2001 n 1 ) ( 2000 + n 1 0 ) ( 2000 + n 1 1 ) ( 2000 + n 1 n 1 ) ) M(n) \; = \; \left( \begin{array}{cccc} \binom{2000}{0} & \binom{2000}{1} & \cdots & \binom{2000}{n-1} \\ \binom{2001}{0} & \binom{2001}{1}& \cdots & \binom{2001}{n-1} \\ \vdots & \vdots & \ddots & \vdots \\ \binom{2000+n-1}{0} & \binom{2000+n-1}{1} & \cdots & \binom{2000+n-1}{n-1} \end{array} \right) then, since ( n + 1 k + 1 ) ( n k + 1 ) = ( n k ) \binom{n+1}{k+1} - \binom{n}{k+1} = \binom{n}{k} we see that, subtracting the ( n 1 ) (n-1) th row from the n n th, then the ( n 2 ) (n-2) nd from the ( n 1 ) (n-1) th, the ( n 3 ) (n-3) rd from the n 2 ) n-2) nd, and so on up, finally subtracting the 1 1 st row from the 2 2 nd, that d e t M ( n ) = d e t ( 1 ( 2000 1 ) ( 2000 n 1 ) 0 M ( n 1 ) 0 ) = d e t M ( n 1 ) \mathrm{det}\,M(n) \; = \; \mathrm{det}\,\left( \begin{array}{cccc} 1 & \binom{2000}{1} & \cdots & \binom{2000}{n-1} \\ 0 & & & \\ \vdots & & M(n-1) & \\ 0 & & & \end{array}\right) \; =\; \mathrm{det}\,M(n-1) and hence d e t M ( 19 ) = d e t M ( 1 ) = 1 \mathrm{det}\,M(19) = \mathrm{det}\,M(1) = \boxed{1} .

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