What does ! above = mean Can someone please explain what the symbol $\stackrel{!}{=}$, consisting of an exclamation mark (!) above an equals sign (=) means? Below is the example I'm trying to decipher:

The normalization factor is chosen such that in average, Dynamic Θ Time passes as fast as physical time. In practice it is determiend by the condition that the interval in Dynamic Θ Time corresponding to a 4-year reference period [$T_0$, $T_1$] should be of exactly the same length:
$T_1 - T_0 \stackrel{!}{=} \int_{T_0}^{T_1} a(t) dt$
 A: I've seen it used as "has to be equal to"; the typical example is the point in a derivation when we use information we know by some general argument. 
A: In my class I use $f'(x)  \stackrel{!}{=} 0 $ to show that we look for a zero of the derivative of $f$ in order to find a local extremum or a stationary point. Other requirements like the definition of a normalization factor are possible candidates.
A: I propose that we adopt the usage of algebraic chess annotations, where the exclamation point ! indicates a particularly good move, which is also suprising or unexpected. 
In mathematics, $\stackrel{!}{=}$ should denote a useful, important, but unexpected equality.
A: In my classes I use it to indicate anxiety.  So an equals with ! over it means "we want to show this equality is true".  An equals without ! means "I am asserting this is true". 
I don't know how universal this convention is, though.  I do know I'm not the only person to use this convention. 
A: I would guess it means the same thing as the slightly more common $\stackrel{.}{=}$ or $\stackrel{\text{def}}{=}$ --- that the two sides of the equation are equal by definition.
