More fun in 2016, Part 2

S = k = 0 504 ( 2016 4 k ) S=\sum_{k=0}^{504}{2016 \choose 4k} Since this was so much fun, let's do it again. The sum S S is of the form S = 2 a + 2 b S=2^a+2^b for two integers a a and b b . Find a + b a+b .


The answer is 3021.

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1 solution

Alan Yan
Oct 5, 2015

Let f ( x ) = ( 1 + x ) 2016 f(x) = (1+x)^{2016} .

Let ω = i \omega = i , the primitive fourth root of unity.

Thus it is easy to see that f ( 1 ) + f ( ω ) + f ( ω 2 ) + f ( ω 3 ) = 4 ( 2016 4 k ) + ( 1 + ω + ω 2 + ω 3 ) ( 2016 4 k + 1 ) + ( 2 + 2 ω 2 ) ( 2016 4 k + 2 ) + ( 1 + ω + ω 2 + ω 3 ) ( 2016 4 k + 3 ) = 4 ( 2016 4 k ) = 4 P 2 2016 + ( 1 + i ) 2016 + ( 1 i ) 2016 = 4 P 2 2016 + ( 2 i ) 1008 + ( 2 i ) 1008 = 4 P 2 2016 + 2 1009 = 4 P P = 2 2014 + 2 1007 2014 + 1007 = 3021 \begin{aligned} f(1) + f(\omega) + f(\omega^2)+f(\omega^3) & = 4\sum{2016 \choose 4k} + (1 + \omega + \omega^2 + \omega^3)\sum{2016 \choose 4k+1} + (2+2\omega^2)\sum{{2016 \choose 4k+2}}+(1 + \omega + \omega^2 + \omega^3)\sum{2016 \choose 4k+3} \\ & = 4\sum{2016 \choose 4k} = 4P \\ 2^{2016} + (1+i)^{2016} + (1-i)^{2016} & = 4P \\ 2^{2016} + (2i)^{1008} + (-2i)^{1008} & = 4P \\ 2^{2016} + 2^{1009} & = 4P \implies P = 2^{2014} + 2^{1007} \implies 2014 + 1007 = \boxed{3021} \end{aligned}

Yes, exactly... you are using the "roots of unity filter" very nicely. Next, do you feel like doing the bonus question to this one ?

Otto Bretscher - 5 years, 8 months ago

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