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I recently answered a question using the following code

$$\begin{array}
\mathbb P\Big(X = \mu+\frac12\left(\gamma+\sqrt{4\kappa-3\gamma^2}\right)\sigma\Big) &=& \dfrac{\sqrt{4\kappa-3\gamma^2}-\gamma}{2\sqrt{4\kappa-3\gamma^2}(\kappa-\gamma^2)} \\
\mathbb P\Big(X = \mu\Big) &=& 1-\dfrac{1}{(\kappa-\gamma^2)} \\
\mathbb P\Big(X = \mu+\frac12\left(\gamma-\sqrt{4\kappa-3\gamma^2}\right)\sigma\Big) &=& \dfrac{\sqrt{4\kappa-3\gamma^2}+\gamma}{2\sqrt{4\kappa-3\gamma^2}(\kappa-\gamma^2)}\end{array}$$ 

and saw a difference in the $P$ and $\mathbb P$ at the start of the first line of the array compared to the second and third, looking like this trimmed image

enter image description here

In full

$$\begin{array} \mathbb P\Big(X = \mu+\frac12\left(\gamma+\sqrt{4\kappa-3\gamma^2}\right)\sigma\Big) &=& \dfrac{\sqrt{4\kappa-3\gamma^2}-\gamma}{2\sqrt{4\kappa-3\gamma^2}(\kappa-\gamma^2)} \\ \mathbb P\Big(X = \mu\Big) &=& 1-\dfrac{1}{(\kappa-\gamma^2)} \\ \mathbb P\Big(X = \mu+\frac12\left(\gamma-\sqrt{4\kappa-3\gamma^2}\right)\sigma\Big) &=& \dfrac{\sqrt{4\kappa-3\gamma^2}+\gamma}{2\sqrt{4\kappa-3\gamma^2}(\kappa-\gamma^2)}\end{array}$$

What might have caused this?

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1 Answer 1

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The error is that the array environment has a required argument that gives information about how the columns should be aligned, and in your case, the initial \mathbb is read as that argument. The characters that are allowed for alignments are c, r, and l (for center, right, and left), plus | and : for vertical lines and dots. Since none of the characters in \mathbb are valid, they are ignored. But the effect is that the \mathbb is eaten by the array environment, and so it is not applied to the P.

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    $\begingroup$ So I should have started with something like $$\begin{array}{lcl} \mathbb P .... Fixed now. Many thanks. $\endgroup$
    – Henry
    Dec 19, 2023 at 2:36
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    $\begingroup$ You can also leave the input empty like $$\begin{array}{} \mathbb P as long as you have the extra brackets $\endgroup$ Dec 20, 2023 at 8:58

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