I What is the purpose of this symbol?

1. May 30, 2016

What exactly is the name and operation of the symbol that is found right after the approximately equal sign in the attachment? I have come across it a lot in readings, and I feel like I should clear up what it is.

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2. May 30, 2016

micromass

Staff Emeritus
The imaginary part?

3. May 30, 2016

Staff: Mentor

What book were you studying? That may give us a clue.

4. May 30, 2016

bahamagreen

The symbol in question is German upper case letter J

On "the approximately equal sign"... watch Carl Bender @ 50:10 - 51:10

5. May 31, 2016

pwsnafu

Dexitify gives $\Im$. German J looks like $\mathfrak{J}$.
Agree with micromass. The imaginary part makes sense contextually as well (approximating the derivative along the imaginary axis).