# how to label equations in this way?

Suppose Alice has an equation labelled by (3). After several pages, she wants to label a new equation by (3)', to indicate that the new equation (3)' is an analog of (3). How does she type in Latex to get the label (3)'?

• "Go ask Alice...when she's ten feet tall." (sorry, I couldn't resist the classic song lyric). – Steven B. Segletes Jun 30 '14 at 11:37
• @StevenB.Segletes - We seriously risk betraying our age by providing such references. :-) I recently managed to lose my audience with a reference to "One Word: Plastics"... – Mico Jun 30 '14 at 12:13
• @Mico And here I discover a little more about “Plastics”. But remember that it's not only the age, also the country we are from influences in our understanding ;) – Manuel Jun 30 '14 at 12:26
• @StevenB.Segletes - I cannot believe that this is the first time that I notice the existence of the lyric... Thanks for the happy accidence! – kwgl Jun 30 '14 at 12:37
• @Manuel - I'm afraid the query to which you had posted one one-word comment and I had posted "Plastics" got deleted before I had a chance to provide some follow-up. Anyway, do a search for "graduate plastics" on youtube and enjoy the scene from the movie "The Graduate", Dustin Hoffman's break-out performance that made him a huge star. :-) – Mico Jun 30 '14 at 12:47

I just came up with a solution to my own problem. Suppose Alice uses \label{eq3} to label (3). She can label a new equation as (3)' in the following way:
$$\tag*{(\ref{eq3})'}\label{eq3'} THE NEW EQUATION.$$

• If you use the amsmath package then you can reference equations with \eqref{}, which puts parentheses around the equation number automatically. – Ubiquitous Jun 30 '14 at 15:02
• Yes. Thank you for leading me to the command! To use (\ref{...}) was one of my primordial habits. Now \eqref is ubiquitous. :) – kwgl Jun 30 '14 at 20:52