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Perimeter and area of the Koch snowflake in the limit

Question

Start with an equilateral triangle of side ss. At each step, on the middle third of every edge erect an outward equilateral triangle whose side is one-third of that edge, and remove the middle third. Repeating forever produces the Koch snowflake. What happens to the perimeter, and what is the limiting enclosed area?

Short answer

The perimeter is infinite; the area tends to 85\tfrac85 of the original triangle, i.e. 235s2\tfrac{2\sqrt3}{5}s^2. Each step multiplies the perimeter by 43\tfrac43, so it diverges. Step kk adds 3⋅4k−13\cdot4^{k-1} triangles each with area 9−k9^{-k} of the original A0=34s2A_0=\tfrac{\sqrt3}{4}s^2, so the total added is A03∑k≥0(4/9)k=35A0\tfrac{A_0}{3}\sum_{k\ge0}(4/9)^k=\tfrac35A_0.

Adapted from Wilmott, Frequently Asked Questions in Quantitative Finance, ch. 10 brainteaser 'Snowflake', p.354; question and answer rewritten by SelfMadeQuant.com.

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