Will you integrate it 2015 times? - III

Calculus Level 2

A function f : R R f : \mathbb{R} \rightarrow \mathbb{R} is differentiated 2015 times to get g ( x ) = d 2015 d x 2015 [ f ( x ) ] g(x) = \dfrac{ d^{2015} }{dx^{2015} } \big[ f(x) \big] .

True or False

If g ( x ) = 2015 g(x) = 2015 for all values of x x , then f ( x ) = 0 f(x) = 0 has finitely many solutions.


This problem is part of the set - Will you integrate it 2015 times? .
False True

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1 solution

Pranshu Gaba
Nov 11, 2015

Let f ( n ) ( x ) f ^{ (n) } (x) denote the n n th derivative of f ( x ) f(x) with respect to x x .

We are given that g ( x ) = f ( 2015 ) ( x ) = 2015 g(x) = f^{(2015)} (x) = 2015 for all x x . After integrating 2015 2015 times, we get

f ( x ) = 2015 2015 ! x 2015 + c 1 x 2014 + c 2 x 2013 + + c 2014 x + c 2015 f(x) = \frac{2015}{ 2015!} x^{2015} + c_{1} x^{2014} + c_{2} x^{2013} + \cdots + c_{2014} x + c_{2015}

Here, c i c_{i} 's can be any real numbers. Since f ( x ) f(x) is a polynomial of degree 2015 2015 , f ( x ) = 0 f(x) = 0 can have at most 2015 2015 real solutions. This means f ( x ) = 0 f(x) = 0 has finitely many solutions, therefore the statement in the problem is True \boxed {\text{True} } . _\square

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