Create a "claim" environment

Question

I am writing a quite long proof in a LaTeX file and I would like to create a "claim" environment to put inside said proof. Can someone help me to understand how this can be done, or at least redirect me to some manual or other resource where it is explained?

What I would like precisely is two environments:

claim: similar to theorem, but maybe instead of getting Claim (bold), only underlined.

claimproof: similar to the proof environment, but with a black box at the end, to distinguish its end from the end of the full proof of the theorem.

Answer 1

Requires amssymb for the black square.

\newenvironment{claim}[1]{\par\noindent\underline{Claim:}\space#1}{}
\newenvironment{claimproof}[1]{\par\noindent\underline{Proof:}\space#1}{\hfill $\blacksquare$}

You can edit and use Claim's proof for example, or anything else. Also, adjust the space above and below, if you want.

\begin{claim}
foo
\end{claim}

\begin{claimproof}
foo
\end{claimproof}

ps: Following the suggestion by @barbara beeton, to avoid new lines before the black square, replace the last pair of {} by

{\leavevmode\unskip\penalty9999 \hbox{}\nobreak\hfill\quad\hbox{$\blacksquare$}}