Funktion

Algebra Level 4

Q Q is a rational function with x Q ( x + 2018 ) = ( x 2018 ) Q ( x ) xQ(x+2018)=(x-2018)Q(x) for all x ( 0 , 2018 ) x ∉ (0, 2018) . If Q ( 1 ) = 1 Q(1)=1 , what is Q ( 2017 ) Q(2017) ?


The answer is 2017.

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1 solution

Patrick Corn
Feb 9, 2021

Let R ( x ) = ( x 2018 ) Q ( x ) . R(x) = (x-2018)Q(x). Then R R is also a rational function and R ( x ) = R ( x + 2018 ) . R(x) = R(x+2018). Nonconstant rational functions are not periodic, so R R is constant. Since R ( 1 ) = 2017 , R(1) = -2017, we get ( x 2018 ) Q ( x ) = 2017 , (x-2018)Q(x) = -2017, so Q ( x ) = 2017 2018 x . Q(x) = \frac{2017}{2018-x}. Then Q ( 2017 ) = 2017 . Q(2017) = \fbox{2017}.

(To see why nonconstant rational functions aren't periodic, consider R ( x ) = g ( x ) / h ( x ) , R(x) = g(x)/h(x), and suppose R R is periodic. If g g is nonconstant, then the number of zeroes of R ( x ) R(x) is finite and nonzero, which is impossible for a periodic function; ditto for the number of zeroes of the periodic function 1 / R ( x ) 1/R(x) if h h is nonconstant.)

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