High Level Determinant

Algebra Level 5

Let A A be a matrix defined by ( α ( i , j ) ) 1 i 2017 , 1 j 2017 (\alpha(i,j)) _{1\leq i \leq 2017, 1 \leq j \leq 2017} where α ( i , j ) = α ( i 1 , j ) ( c i + j + 1 ) 1 i 2017 , 1 j 2017 \alpha(i,j)=\alpha(i-1,j)\cdot(c-i+j+1)_{1\leq i \leq 2017, 1\leq j \leq 2017} in which α ( 1 , j ) = 1 \alpha(1,j)=1 for all values of j j ; and c = 2017 c=2017 . Find the largest values for n n , such that n ! n! divides the determinant d d , where d d represents the determinant of A A .


The answer is 2016.

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1 solution

Mark Hennings
Nov 26, 2018

We solve the recurrence relation for the coefficients of the matrix A A to write α ( u , v ) = ( c + v 1 ) ! ( c + v u ) ! 1 u , v c \alpha(u,v) \; = \; \frac{(c+v-1)!}{(c+v-u)!} \hspace{2cm} 1 \le u,v \le c and hence we deduce that α ( u , v + 1 ) α ( u , v ) = { ( u 1 ) α ( u 1 , v ) 2 u c , 1 v c 1 0 u = 1 , 1 v c 1 \alpha(u,v+1) - \alpha(u,v) \; = \; \left\{ \begin{array}{lll} (u-1)\alpha(u-1,v) & \hspace{1cm} & 2 \le u \le c\,,\,1 \le v \le c-1 \\ 0 & & u = 1 \,,\, 1 \le v \le c-1 \end{array}\right. For each 1 K c 1 \le K \le c , let M ( K ) M(K) be the K × K K \times K matrix whose coefficients are α ( u , v ) \alpha(u,v) for 1 u , v K 1 \le u,v \le K . We have shown above that, if we apply suitable column operations to the matrix, that d e t M ( K ) = 1 0 0 0 α ( 2 , 1 ) α ( 1 , 1 ) α ( 1 , 2 ) α ( 1 , K 1 ) α ( 3 , 1 ) 2 α ( 2 , 1 ) 2 α ( 2 , 2 ) 2 α ( 2 , K 1 ) α ( 4 , 1 ) 3 α ( 3 , 1 ) 3 α ( 3 , 2 ) 3 α ( 3 , K 1 ) α ( K , 1 ) ( K 1 ) α ( K 1 , 1 ) ( K 2 ) α ( K 1 , 2 ) ( K 1 ) α ( K 1 , K 1 ) = α ( 1 , 1 ) α ( 1 , 2 ) α ( 1 , K 1 ) 2 α ( 2 , 1 ) 2 α ( 2 , 2 ) 2 α ( 2 , K 1 ) 3 α ( 3 , 1 ) 3 α ( 3 , 2 ) 3 α ( 3 , K 1 ) ( K 1 ) α ( K 1 , 1 ) ( K 2 ) α ( K 1 , 2 ) ( K 1 ) α ( K 1 , K 1 ) = ( K 1 ) ! d e t M ( K 1 ) \begin{aligned} \mathrm{det}\,M(K) & = \; \left| \begin{array}{ccccc} 1 & 0 & 0 & \cdots & 0 \\ \alpha(2,1) & \alpha(1,1) & \alpha(1,2) & \cdots & \alpha(1,K-1) \\ \alpha(3,1) & 2\alpha(2,1) & 2\alpha(2,2) & \cdots & 2\alpha(2,K-1) \\ \alpha(4,1) & 3\alpha(3,1) & 3\alpha(3,2) & \cdots & 3\alpha(3,K-1) \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ \alpha(K,1) & (K-1)\alpha(K-1,1) & (K-2)\alpha(K-1,2) & \cdots & (K-1)\alpha(K-1,K-1) \end{array}\right| \\ & = \left| \begin{array}{cccc} \alpha(1,1) & \alpha(1,2) & \cdots & \alpha(1,K-1) \\ 2\alpha(2,1) & 2\alpha(2,2) & \cdots & 2\alpha(2,K-1) \\ 3\alpha(3,1) & 3\alpha(3,2) & \cdots & 3\alpha(3,K-1) \\ \vdots & \vdots & \ddots & \vdots \\ (K-1)\alpha(K-1,1) & (K-2)\alpha(K-1,2) & \cdots & (K-1)\alpha(K-1,K-1) \end{array}\right| \; = \; (K-1)!\mathrm{det}\,M(K-1) \end{aligned} and hence d e t M ( K ) = j = 1 K 1 j ! \mathrm{det}\,M(K) \; = \; \prod_{j=1}^{K-1} j! so that d = A = j = 1 2016 j ! d \; = \; |A| \; = \; \prod_{j=1}^{2016} j! Thus 2016 ! 2016! certainly divides d d , but the prime 2017 2017 does not divide d d , so that 2017 ! 2017! does not divide d d . Thus the answer is 2016 \boxed{2016} .

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