A = r = 0 ∑ 2 0 1 6 exp ( 2 0 1 7 i 2 0 1 6 π r 2 ) , B = r = 0 ∑ 2 0 1 5 exp ( − 2 0 1 6 i 2 0 1 7 π r 2 )
Given that A and B are defined as above, ℜ ( B A ) can be written as q p for coprime positive integers p and q . Evaluate p + q .
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(+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 = z 1 n = − ∞ ∑ ∞ e − π n 2 / z put z = p 2 i q + ϵ and let ϵ → 0 . We get p 1 n = 0 ∑ p − 1 exp ( p 2 π i n 2 q ) = 2 q e π i / 4 n = 0 ∑ 2 q − 1 exp ( − 2 q π i n 2 p ) .
Can you please tell me a good book to study Gauss sum? Thanks!
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If we consider the Gauss sum G ( a , c ) = r = 0 ∑ c − 1 e x p ( c 2 π i a r 2 ) then we can use standard identities concerning Gauss sums to deduce that A = G ( 1 0 0 8 , 2 0 1 7 ) = ϵ 2 0 1 7 2 0 1 7 ( 2 0 1 7 1 0 0 8 ) = 2 0 1 7 ( 2 0 1 7 1 0 0 8 ) where ϵ m = 1 if m ≡ 1 ( m o d 4 ) , while ϵ m ≡ i if m ≡ 3 ( m o d 4 ) , and ( q p ) is the Jacobi symbol, while B ⋆ = = = = r = 0 ∑ 2 0 1 5 e x p ( 2 0 1 6 i 2 0 1 7 π r 2 ) = r = 0 ∑ 2 0 1 5 e x p ( 4 0 3 2 2 π i 2 0 1 7 r 2 ) 2 1 [ r = 0 ∑ 2 0 1 5 e x p ( 4 0 3 2 2 π i 2 0 1 7 r 2 ) + r = 0 ∑ 2 0 1 5 e x p ( 4 0 3 2 2 π i 2 0 1 7 ( 2 0 1 6 + r ) 2 ) ] 2 1 r = 0 ∑ 4 0 3 1 e x p ( 4 0 3 2 2 π i 2 0 1 7 r 2 ) = 2 1 G ( 2 0 1 7 , 4 0 3 2 ) 2 1 ( 1 + i ) ϵ 2 0 1 7 − 1 4 0 3 2 ( 2 0 1 7 4 0 3 2 ) = ( 1 + i ) 1 0 0 8 ( 2 0 1 7 1 0 0 8 ) and hence B ⋆ A = 1 + i 1 1 0 0 8 2 0 1 7 so that R e ( B A ) = 2 1 1 0 0 8 2 0 1 7 = 4 0 3 2 2 0 1 7 making the answer 2 0 1 7 + 4 0 3 2 = 6 0 4 9 .