# Conventional letters to use

I have already defined a group of errors of different types: $\omega_a, \omega_b, \ldots, \omega_z$

As well as another group of errors of various types: $\Omega_A, \Omega_B, \ldots, \Omega_Z$.

Now I am looking for symbols or letters to represent any error of the two groups. There are two ways I could think of

1) "let $\omega$ denote any error of the first group without specifying the error type" and "let $\Omega$ denote any error of the second group without specifying the error type"...

2) let $\omega = \{\omega_a, \omega_b, \ldots, \omega_z\}$ and $\Omega = \{\Omega_a, \Omega_b, \ldots, \Omega_z\}$, but in this case, I do not know which letters to use to represent an error without saying its type.

Could anyone tell me which way is better? Thank you.

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1) looks fine to me. I do analogous things fairly frequently (e.g. with $\lambda_1, ... \lambda_n$ being eigenvalues, letting $\lambda$ be any of the eigenvalues). – Qiaochu Yuan Aug 10 '11 at 14:51
This is not exactly a math question... :) – Mariano Suárez-Alvarez Aug 10 '11 at 14:52

You may use the following: "If we wish to leave the type of an error of the first (respectively second) group unspecified, we denote the error by $\omega$ (respectively $\Omega$)''.
Though, similarly to your first suggestion, you may also use: "We let $\omega$ (respectively $\Omega$) denote an error of the first (respectively second) group, without specifying the error type''. – Shai Covo Aug 10 '11 at 15:41