A question of degree

Let f ( x ) f(x) be the unique polynomial that satisfies

f ( n ) = i = 1 n i 23 , for all positive integers n . f(n) = \sum_{i=1}^n i^{23 }, \mbox{ for all positive integers } n.

What is the degree of f ( x ) f(x) ?


The answer is 24.

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3 solutions

we all know that a sequence with a constant common difference, the general term is a one degree polynomial and the sum is n ( n + 1 ) 2 \frac{n(n+1)}{2} and it is a 2 degree polynomial. and the general term of a sequence with an increasing common difference is a two degree polynomial. the sum is n ( n + 1 ) ( 2 n + 1 ) 6 \frac{n(n+1)(2n+1)}{6} and it is a 3 degree polynomial. so we can conclude that the degree of the polynomial sum of a 23 degree polynomial general term is just by adding 1 degree to it. so it is 24.

Moderator note:

Good approach using the Method of differences , to rigorously determine the degree of the polynomial.

I didn't get it. can you help me out?

Ayush Jain - 7 years, 10 months ago

Well, Pranav A, your method is also good, as I also couldn't think of any other method than method of differences. Also, the solution of method of differences uses inductive reasoning to conclude that the following pattern also holds for degree 23. But your method uses deductive reasoning. Hats off!

Utkarsh Mehra - 7 years, 9 months ago

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Thank you! :)

Pranav Arora - 7 years, 9 months ago

Great solution! It almost took me a week to solve this problem and what I ended up with is a very long method. This is very compact and nicely done. Thanks for sharing! :)

Pranav Arora - 7 years, 9 months ago

use method of dumb luck

math man - 6 years, 7 months ago

wonderful.

Shuaib Tarek - 7 years, 10 months ago
Pranav Arora
Aug 17, 2013

It is given that f(x) is a polynomial.

Let f ( x ) = i = 0 p a i x i \displaystyle f(x)=\sum_{i=0}^p a_ix_i where a p 0 a_p \neq 0 .

From the question, we have, f ( x ) f ( x 1 ) = x 23 f(x)-f(x-1)=x^{23}

Substituting f ( x ) f(x) and f ( x 1 ) f(x-1) , we notice that the highest power of x x remaining on LHS is p 1 p-1 . Notice that a p x p a_p x^p cancels out.

Comparing both the sides, we find that p 1 = 23 p = 24 p-1 =23 \Rightarrow p=24 .

Therefore, the degree of f ( x ) f(x) is 24.

Santanu Banerjee
Aug 13, 2013

Recollecting sum of N natural numbers = N ( N + 1 ) / 2..............................

.Degree 1 higher than that of what we are summing

Sum of the series ( N ^ 2 ) where N starts from 1 is N ( N +1 ) ( 2 N + 1 ) / 6........................

Degree 1 higher than that of what we are summing

Sum of the series ( N ^ 3 ) where N starts from 1 is N ( N +1 ) N ( N + 1 ) / 4........................

Degree 1 higher than that of what we are summing

Thus here too the degree should be one higher than 23 which is 24

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