Capital and small nu

Question

I couldn't figure out the command for Capital Nu as shown in the picture. I tried \Nu but it doesn't work.

I googled but found nothing helpful.

Answer 1

It is certainly a math calligraphic V. Here is the formula, reproduced as faithfully as possible:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\newcommand\I{\mathrm i}
\newcommand\E{\mathrm e}

\begin{document}

\begin{alignat*}{2}
& \widetilde{\mathcal{V}} &{} = -\hbar \Omega \Bigl\{ &\sigma_ + \E^{-\I(\Delta t-\varphi)}\bigl[1 + \I\eta(b\E^{-\I\nu t} + b^{\dagger}\E^{\I\nu t})\bigr]\Bigr. \\
  & & \Bigl. {}+{} &\sigma_ -\E^{\I(\Delta t-\varphi)}\bigl[1 - \I\eta(b\E^{-\I\nu t} + b^{\dagger}\E^{\I\nu t})\bigr]\Bigr\}. 
 \end{alignat*}%

\end{document} 

Answer 2

Some Greek capital letters look the same as Latin capitals and therefore don't have unique commands. Nu is one of them. The list of these Greek capitals is available from several sources, including

http://www.mathworks.com/matlabcentral/answers/14751-greek-alphabet-and-latex-commands-not-a-question and https://tex.stackexchange.com/a/49602/61108

From the linked TeX.SX answer the list is

Α α alpha
Β β beta
Ε ε epsilon
Ζ ζ zeta
Η η eta
Ι ι iota
Κ κ kappa
Μ μ mu
Ν ν nu
Ο ο omicron
Ρ ρ rho
Τ τ tau
Υ υ upsilon
Χ χ chi

Just use N for capital nu.