# What is the Modulus of an Eigenvalue?

1. May 18, 2006

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2. May 18, 2006

### LeonhardEuler

The modulus of a real number is its absolute value. Since this is posted under quantum mechanics, I am assuming the the eigenvalue is real. In a more general case, though, the modulus of a complex number, a + bi, is $\sqrt{a^2+b^2}$.

3. May 19, 2006

Staff Emeritus
Yes, if you regard a complex number as a vector in the plane (Feynmann;s "little arrows") then its modulus is its length. This obviously agrees with LeonhardEuler's algebraic definition.

4. May 19, 2006

### masudr

I'd credit the arrows to Argand, not Feynman...

5. May 19, 2006