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HMMT 2025 A8

  • Algebra and Number Theory
  • Answer

Define sgn⁡(x)\operatorname {sgn}(x) to be 1 when xx is positive, −1-1 when xx is negative, and 0 when xx is 0. Compute

∑n=1∞sgn⁡(sin⁡(2n))2n.\sum_{n = 1}^{\infty}\frac{\operatorname{sgn}(\sin(2^{n}))}{2^{n}}.

(The arguments to sin are in radians.)

Dataset text NuminaMath olympiads. Quoted with source.

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\frac{1 - \frac{2}{\pi}}{2}

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