Volumes 1

Geometry Level 3

For k > 0 k>0 , let R k R_k be the region in R 3 \mathbb{R}^3 given by

R k = { ( x , y , z ) : x k + y k 1 AND y k + z k 1 AND z k + x k 1 } . R_k = \{(x,y,z) : \left|x\right|^k + \left|y\right|^k \leq 1 \text{ AND } \left|y\right|^k + \left|z\right|^k \leq 1 \text{ AND } \left|z\right|^k + \left|x\right|^k \leq 1\}.

Let V k V_k be the volume of R k R_k . Let V = lim k V k V_{\infty} = \lim_{k\rightarrow\infty}{V_k} . There exists integers a , b , c a,b,c such that

lim k k a ( V V k ) = b π c . \lim_{k\rightarrow\infty}{k^a (V_{\infty} - V_{k})} = b \pi^c.

Submit a + b + c + V 1 + 10 V 1 / 2 + ( V 2 16 ) 2 a+b+c+V_1+10 V_{1/2}+(V_2 - 16)^2 .


The answer is 137.

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1 solution

I got V 1 = 2 , V 1 / 2 = 3 / 10 , V 2 = 16 8 2 , V = 8 V_1 = 2, V_{1/2} = 3/10, V_2 = 16-8\sqrt{2}, V_{\infty} = 8 (these are pretty easy). I also got

V V k 8 3 ( 1 Γ ( 1 + 1 k ) 2 Γ ( k + 2 k ) ) 8 3 π 2 6 k 2 V_{\infty} - V_k \sim 8\cdot3 \Bigg(1-\frac{\Gamma \left(1+\frac{1}{k}\right)^2}{\Gamma \left(\frac{k+2}{k}\right)}\Bigg) \sim 8\cdot3\frac{\pi^2}{6}k^{-2}

but I am not 100% sure this is correct (I may have made a mistake).

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