pellman
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Looks like \triangleq. I first came across it on the wikipedia article for Lie derivative. http://en.wikipedia.org/wiki/Lie_derivative#The_Lie_derivative_of_a_function
The triangle equals sign, represented as \triangleq, is used to denote "is defined as" or "equals by definition," particularly in mathematical contexts such as the Lie derivative. This notation was highlighted in a discussion referencing its appearance in the Wikipedia article on Lie derivatives. In contrast, the triple-equals sign, \equiv, signifies that a function identically equals a constant or another function, indicating a stronger form of equality.
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