I want to introduce a notation for concatenating vectors in a manuscript that I represent vectors with bold characters, $\textbf{x}$. The problem is defining it like $\textbf{x}:\textbf{y}$ introduces an extra distance between two characters which is far more than $\textbf{x:y}$. On the other hand the latter is not what I am looking for since I need non-bold subscripts for vectors and I need to use two $\textbf$ environments. Any suggestions?

This is bad concatenation $\textbf{x}:\textbf{y}$
This is good concatenation $\textbf{x:y}$
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    Enclose the : itself in braces (to prevent it from being treated as a binary operator, which causes the space). – Paul Gessler Jul 4 '14 at 13:20
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    : is a considered a relation symbol; $\mathbf{x}{:}\mathbf{y}$ is what you need. – egreg Jul 4 '14 at 13:20

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 %uncomment to make `:` act like `{:}` by default.




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    I'm sure \mathbf should be used; or the symbols could turn out to be in bold italic, if the formula is inside a theorem statement. – egreg Jul 4 '14 at 13:24
  • @Naji If you only need : in that context and never as a relation symbol, I'm sure David will be glad to add a way for inputting : directly. – egreg Jul 4 '14 at 13:27
  • I am actually quite satisfied with the responses. Although now what seems puzzling to me is that using $\bm$ keeps the characters in their variable format but usually vectors are represented with normal text format. The question is what to do to handle the case where in a theorem environment we would get normal format. – Naji Jul 4 '14 at 13:31
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    @egreg done: -) – David Carlisle Jul 4 '14 at 13:32
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    @Naji The whole point of bm is to give the bold version of a character so if it was in math italic it gives bold math italic. If you want upright bold then \mathbf will give that – David Carlisle Jul 4 '14 at 13:33

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