Determinants

Algebra Level 4

What is the determinant of the matrix:

( 1 2016 2 2016 3 2016 201 8 2016 2 2016 3 2016 4 2016 201 9 2016 3 2016 4 2016 5 2016 202 0 2016 201 8 2016 201 9 2016 202 0 2016 403 5 2016 ) \begin{pmatrix} 1^{2016} & 2^{2016} & 3^{2016} & \ldots & 2018^{2016} \\ 2^{2016} & 3^{2016} & 4^{2016} & \ldots & 2019^{2016} \\ 3^{2016} & 4^{2016} & 5^{2016} & \ldots & 2020^{2016} \\ \vdots & \vdots & \vdots & & \vdots\\ 2018^{2016} & 2019^{2016} & 2020^{2016} & \ldots & 4035^{2016} \\ \end{pmatrix}


The answer is 0.

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1 solution

Mark Hennings
Oct 18, 2017

For any n 2 n \ge 2 the collection of polynomials x n 1 , ( x + 1 ) n 1 , ( x + 2 ) n 1 , . . . , ( x + n ) n 1 x^{n-1},(x+1)^{n-1},(x+2)^{n-1},...,(x+n)^{n-1} form a set of n + 1 n+1 polynomials in the n n -dimensional vector space of polynomials of degree at most n 1 n-1 . Thus they form a linearly dependent collection, and so there exists a collection of constants a 0 , a 1 , a 2 , . . . , a n a_0,a_1,a_2,...,a_n (not all zero) such that F ( x ) a 0 x n 1 + a 1 ( x + 1 ) n 1 + a 2 ( x + 2 ) n 1 + . . . + a n ( x + n ) n 1 0 F(x) \; \equiv \; a_0 x^{n-1} + a_1(x+1)^{n-1} + a_2(x+2)^{n-1} + ... + a_n(x+n)^{n-1} \; \equiv \; 0 If A ( n ) A(n) is the ( n + 1 ) × ( n + 1 ) (n+1)\times(n+1) matrix A ( n ) i , j = ( i + j 1 ) n 1 1 i , j n + 1 A(n)_{i,j} \; = \; (i+j-1)^{n-1} \hspace{2cm} 1 \le i,j \le n+1 then A ( n ) ( a 0 a 1 a n ) = ( F ( 1 ) F ( 2 ) F ( n + 1 ) ) = 0 A(n) \left( \begin{array}{c} a_0 \\ a_1 \\ \vdots \\ a_n \end{array} \right) \; = \; \left( \begin{array}{c} F(1) \\ F(2) \\ \vdots \\ F(n+1) \end{array} \right) \; = \; \mathbf{0} and hence the matrix A ( n ) A(n) is singular, and has determinant 0 \boxed{0} . In this case, we see that A ( 2017 ) A(2017) has determinant 0 \boxed{0} .

In fact, this solution generalizes immediately to any n × n n\times n matrix where the ( i , j ) (i,j) term is ( a i + b j 1 ) m (a_i+b_j-1)^m with m n 2 m \leq n-2 . This can produce interesting results, esp when we have variables / fractions.

E.g What is the determinant of the matrix:

( 2 x x + y x + z x + y 2 y y + z x + z y + z 2 z ) \begin{pmatrix} 2x & x+y & x+z \\ x+y & 2y & y+z\\ x+z & y+z & 2z \\ \end{pmatrix}

It is not immediately obvious that the rows are linearly independent, but since we know the determinant is 0, hence they are!

Calvin Lin Staff - 3 years, 7 months ago

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Indeed, except I think you mean ( n + 1 ) × ( n + 1 ) (n+1)\times(n+1) matrix, or else m n 2 m \le n-2 .

Mark Hennings - 3 years, 7 months ago

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Ah yes. Details .... Thanks!

Calvin Lin Staff - 3 years, 7 months ago

Sir @Mark Hennings I couldn't get how the set of polynomials form a linearly independent collection.Can you please explain in more details.

Navin Murarka - 3 years, 7 months ago

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The vector space of polynomials of degree at most n 1 n-1 has dimension n n (the monomials 1 1 , x x , x 2 x^2 , ..., x n 1 x^{n-1} form a basis, for example). Since there are n + 1 n+1 polynomials in the collection x n 1 x^{n-1} , ( x + 1 ) n 1 (x+1)^{n-1} , ..., ( x + n ) n 1 (x+n)^{n-1} , they must be linearly dependent (basically, by the definition of what dimension is).

General results tell us that

  • any linearly independent set can be extended to a basis,
  • any spanning set has a subset which is a basis.

Together, these results tell us that the dimension of a finite-dimensional vector space is well-defined (two bases have the same number of elements), and that any linearly independent set cannot have more elements than the dimension of the vector space. It is this last result that I am using.

Mark Hennings - 3 years, 7 months ago

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Thanks I got it

Navin Murarka - 3 years, 7 months ago

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