Independent terms

Let, T i j k l m n p q , 0 i , j , k , l , m , n , p , q 4 T_{ijklmnpq} ,0\leq i,j,k,l,m,n,p,q\leq 4 are the elements of an ( 0 , 8 ) (0,8) Tensor which have certain properties as follows.

  1. T i j k l m n p q = T j i k l m n p q T_{ijklmnpq}=T_{jiklmnpq}

    2 . T i j k l m n p q = T i j l k m n p q T_{ijklmnpq}=T_{ijlkmnpq}

    3 . T i j k l m n p q = T i j k l n m p q T_{ijklmnpq}=T_{ijklnmpq}

    4 . T i j k l m n p q = T k l i j m n p q T_{ijklmnpq}=-T_{klijmnpq}

    5 . T i j k l m n p q = T i j m n k l p q T_{ijklmnpq}=-T_{ijmnklpq}

Let, A A be the number of independent tensor elements of this ( 0 , 8 ) (0,8) tensor. What is the value of A + 100 A+100 ?


The answer is 2020.

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1 solution

Alapan Das
Feb 4, 2020

What we see from the given specialties that we can get 10 10 independent elements for each ( i , j ) ; ( k , l ) ; ( m , n ) (i,j); (k,l); (m,n) from first three properties. Now, say T i j k l m n p q = T α β γ δ T_{ijklmnpq}=T_{{\alpha}{\beta}{\gamma}{\delta}} . Now, α , β , γ \alpha, \beta, \gamma all can have ( 4 4 4 ) 2 + 4 = 10 \frac{(4*4-4)}{2}+4=10 independent points and δ \delta can have 4 4 = 16 4*4=16 . Now, from 4 , 5 4,5 we can see whenever at least two of α , β , γ \alpha, \beta, \gamma meaning, at least any two of ( i , j ) ; ( k , l ) ; ( m , n ) (i,j); (k,l); (m,n) are equal T i j k l m n p q = 0 T_{ijklmnpq}=0 . Hence, we can chose independent points for { i j k l m n ijklmn } in ( 10 3 ) = 120 \binom{10}{3}=120 ways and in additional to this we can choose 16 16 point for p q pq . hence, total 120 16 = 1920 120*16=1920 ways. So, we have total 1920 ways to choose independent components( considering each connection of 4 points of α \alpha , β \beta , γ \gamma , δ \delta an element) for this ( 0 , 8 ) (0,8) tensor T T . hence, our desired answer is 1920 + 100 = 2020 1920+100=2020 .

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