Summing Complex Fractional Exponents

Algebra Level 5

A = r = 0 2016 exp ( i 2016 π r 2 2017 ) , B = r = 0 2015 exp ( i 2017 π r 2 2016 ) \text{A} = \sum_{r=0}^{2016} \exp \left(\frac{i 2016\pi r^2}{2017}\right), \qquad \text{B} = \sum_{r=0}^{2015} \exp \left(-\frac{i 2017\pi r^2}{2016}\right)

Given that A \text{A} and B \text{B} are defined as above, ( A B ) \Re \left( \dfrac {\text{A}}{\text{B}} \right) can be written as p q \sqrt{\dfrac{p}{q}} for coprime positive integers p p and q q . Evaluate p + q p+q .

Clarifications:

  • exp ( x ) = e x \exp (x) = e^x , where e 2.71828 e \approx 2.71828 denotes the Euler's number .
  • i = 1 i = \sqrt{-1} .
  • If a a and b b are real numbers, then ( a + b i ) = a \Re(a+bi) = a .


The answer is 6049.

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1 solution

Mark Hennings
Oct 18, 2016

If we consider the Gauss sum G ( a , c ) = r = 0 c 1 e x p ( 2 π i a r 2 c ) G(a,c) \; = \; \sum_{r=0}^{c-1} \mathrm{exp}\left( \frac{2 \pi i a r^2}{c} \right) then we can use standard identities concerning Gauss sums to deduce that A = G ( 1008 , 2017 ) = ϵ 2017 2017 ( 1008 2017 ) = 2017 ( 1008 2017 ) A \; = \; G(1008,2017) \; =\; \epsilon_{2017} \sqrt{2017} \left( \frac{1008}{2017}\right) \; = \; \sqrt{2017}\left(\frac{1008}{2017}\right) where ϵ m = 1 \epsilon_m = 1 if m 1 ( m o d 4 ) m \equiv 1 \pmod{4} , while ϵ m i \epsilon_m \equiv i if m 3 ( m o d 4 ) m \equiv 3 \pmod{4} , and ( p q ) \left(\frac{p}{q}\right) is the Jacobi symbol, while B = r = 0 2015 e x p ( i 2017 π r 2 2016 ) = r = 0 2015 e x p ( 2 π i 2017 r 2 4032 ) = 1 2 [ r = 0 2015 e x p ( 2 π i 2017 r 2 4032 ) + r = 0 2015 e x p ( 2 π i 2017 ( 2016 + r ) 2 4032 ) ] = 1 2 r = 0 4031 e x p ( 2 π i 2017 r 2 4032 ) = 1 2 G ( 2017 , 4032 ) = 1 2 ( 1 + i ) ϵ 2017 1 4032 ( 4032 2017 ) = ( 1 + i ) 1008 ( 1008 2017 ) \begin{array}{rcl} B^\star & = & \displaystyle \sum_{r=0}^{2015} \mathrm{exp}\left( \frac{i 2017 \pi r^2}{2016}\right) \; = \; \sum_{r=0}^{2015} \mathrm{exp} \left(\frac{2\pi i 2017 r^2}{4032}\right) \\ & = & \displaystyle \frac12\left[ \sum_{r=0}^{2015} \mathrm{exp}\left(\frac{2\pi i 2017 r^2}{4032}\right) + \sum_{r=0}^{2015} \mathrm{exp}\left(\frac{2\pi i 2017(2016+r)^2}{4032}\right)\right] \\ & = & \displaystyle \frac12\sum_{r=0}^{4031} \mathrm{exp}\left(\frac{2\pi i 2017 r^2}{4032}\right) \; = \; \tfrac12G(2017,4032) \\ & = & \displaystyle \tfrac12(1+i)\epsilon_{2017}^{-1}\sqrt{4032}\left(\frac{4032}{2017}\right) \; = \; (1+i)\sqrt{1008}\left(\frac{1008}{2017}\right) \end{array} and hence A B = 1 1 + i 2017 1008 \frac{A}{B^\star} \; = \; \frac{1}{1+i}\sqrt{\frac{2017}{1008}} so that R e ( A B ) = 1 2 2017 1008 = 2017 4032 \mathfrak{Re} \left(\frac{A}{B}\right) \; = \; \tfrac12\sqrt{\frac{2017}{1008}} \; = \; \sqrt{\frac{2017}{4032}} making the answer 2017 + 4032 = 6049 2017 + 4032 = \boxed{6049} .

(+1) Nice solution! I didn't know about Gauss Sums, something new to learn :)

Another way is to use the functional equation of theta function, namely n = e π n 2 z = 1 z n = e π n 2 / z \displaystyle \sum_{n=-\infty}^\infty e^{-\pi n^2 z}=\frac{1}{\sqrt{z}}\sum_{n=-\infty}^{\infty}e^{-\pi n^2/z} put z = 2 i q p + ϵ \displaystyle z = \dfrac{2i q}{p} + \epsilon and let ϵ 0 \epsilon \to 0 . We get 1 p n = 0 p 1 exp ( 2 π i n 2 q p ) = e π i / 4 2 q n = 0 2 q 1 exp ( π i n 2 p 2 q ) \displaystyle \dfrac{1}{\sqrt{p}}\sum_{n=0}^{p-1}\exp \left(\frac{2\pi i n^2 q}{p} \right)=\dfrac{e^{\pi i/4}}{\sqrt{2q}}\sum_{n=0}^{2q-1}\exp \left(-\frac{\pi i n^2 p}{2q} \right) .

Ishan Singh - 4 years, 8 months ago

Can you please tell me a good book to study Gauss sum? Thanks!

Thércyo Cavalcanti - 4 years, 1 month ago

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