What does this equation mean?

Every accelerated detector’s normalized spectrum is the same universal curve g , only rescaled by a/c . For two accelerations separated by a small increment, a and a+$\delta$ a , standard calculus for a scale family gives, to leading order in $\delta$ a , $D_{\rm det}(a,\, a+\delta a) \;\approx\; \left[\max_{u>0} u\,g(u)\right] \cdot \frac{|\delta a|}{a}$. This is the same trick used to derive Wien’s displacement law: because u\,g(u) rises from zero, passes through a single interior maximum, and falls back to zero, the integral of the absolute value of its derivative over all u is exactly twice that maximum, with no need to evaluate the integral directly. Finding the maximum of u\,g(u) = 24\,$u^2$/($e^{2\pi u}$-1) means solving $e^{2\pi u}(1-\pi u)$=1 , the same class of transcendental equation that fixes the peak of a Planck spectrum weighted by an extra power of frequency. Solved by Newton’s method to five-figure precision, the nontrivial root is $u^\ast$ $\approx$ 0.25363 , at which $u^\ast$ g($u^\ast$) $\approx$ 0.3937 . The leading-order sensitivity is therefore