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Value of the continued fraction 2 + 2/(2 + ...)

Question

Find the value of the infinite continued fraction x=2+22+22+⋯.x=2+\cfrac{2}{2+\cfrac{2}{2+\cdots}}.

Short answer

1+31+\sqrt3. The tail repeats the whole expression, so x=2+2/xx=2+2/x, i.e. x2−2x−2=0x^2-2x-2=0. The roots are 1±31\pm\sqrt3, and since x>2x>2 we take 1+3≈2.7321+\sqrt3\approx2.732 (the convergents do approach it).

Adapted from Joshi, Denson & Downes, Quant Job Interview Questions and Answers, Q6.7, p.190; question and answer rewritten by SelfMadeQuant.com.

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