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I just came across this "subset" symbol in a PDF:

$$\Omega \subsetneq T$$

I've never seen it before, and I tried looking for it via Detexify (to no avail). What does it mean?

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This means $\Omega$ is a proper subset of $T$. That is, $\Omega\subseteq T$ but $\Omega \neq T$.

For example, $\{1, 2, 3\}\subsetneq \{1, 2, 3, 4\}$.

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    $\begingroup$ @gablin: Another variant is $\subsetneqq$, and of course some folks use $\subset$ to mean proper subset. $\endgroup$ – Brian M. Scott Jan 17 '12 at 10:23
  • $\begingroup$ Argh, the author mixes the various notations throughout the document! Why not just pick one and be consistent... $\endgroup$ – gablin Jan 17 '12 at 10:28
  • $\begingroup$ Well, people sometimes just use $\subset$ to mean $\subseteq$. Because it is quicker to type, perhaps? Context is all-important! $\endgroup$ – user1729 Jan 17 '12 at 10:39
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$\subsetneq$ ($\text{"\\subsetneq"}$) means: subset, but not equal.

Here's a nice example:

Let Ω be a half-strip in the complex plane: $$ \Omega = \{ z \in \mathbb{C} | x_1 \leq \mathrm{Re} (z) \leq x_2 \text{ and } \mathrm{Im} (z) \geq y_0 \} \subsetneq \mathbb{C}. \, $$

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