15

This number is a composite.

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(F₀¹⁵ + F₁¹⁵ + F₂¹⁵ + F₃¹⁵ + F₄¹⁵) and (F₀¹⁵ + F₁¹⁵ + F₂¹⁵ + F₃¹⁵ + F₄¹⁵ + 6) are sexy primes. Note that the first five Fermat numbers are all prime. [Wesolowski]