I want to know the difference between:

$\hat{\mathbf{M}}$  % hat is not bold
$\mathbf{\hat{M}}$ % hat is bold

What is correct way (bold/normal). If both what are cases when I should use bold and when normal hat.

  • 2
    It depends on whether you want the hat to be bold or not. However the first should be $\hat{\mathbf{M}}$ – egreg Apr 26 '16 at 19:26
  • @Mico, I wanted to know when should I use bold hat and when normal hat. – user31177 Apr 26 '16 at 19:35
  • 2
    Whether or not the hat symbol is supposed to be bold as well really depends on what M, \hat{M}, \mathbf{M}, and thus also \hat{\mathbf{M}} and \mathbf{\hat{M}} are supposed to mean. – Mico Apr 26 '16 at 19:37

It depends on the meaning of what you want to write. This is a matter of pure semantics. You have two options:

  1. If the « hat » means « matrix M », then the hat is not part of the name of the matrix. It just says that M is a matrix. In this case, you should use \hat{\mathbf{M}}.
  2. However, if you call the matrix « M̂ », that is, the « hat » is actually a part of the name, then you should use \mathbf{\hat{M}}.

To sum up: if the « hat » is part of the name, make it bold, else make it normal.

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