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$$\triangleright\quad \text{and}\quad \triangleleft$$

I saw these notations in some abstract algebra texts and i don't know what does this mean.

What do they mean?

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    $\begingroup$ Fortunately in this case the context is narrow enough to take a guess. In general though, if you have a question about notation, give a line or two of context where it is used. After all, not all mathematical symbols have a fixed universal meaning in all contexts. $\endgroup$ – rschwieb Apr 14 '14 at 14:14
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It probably means normal subgroup in the context of groups or ideal in the context of rings.

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