here is a minimal exaple which doesn’t render | directely
line | $\lbrace\begin{array}{l}\theta_1 x_1 + \dots + \theta_n x_n | \theta_1 + \dots + \theta_K =1,\\ x_1, \dots, x_n \in [0,1] \end{array}\rbrace$ | × | ✓ | ✓ |
– | – | – | – | – | –
while replacing | with \mid, it works
line | $\lbrace\begin{array}{l}\theta_1 x_1 + \dots + \theta_n x_n \mid \theta_1 + \dots + \theta_K =1,\\ x_1, \dots, x_n \in [0,1] \end{array}\rbrace$ | × | ✓ | ✓ |
– | – | – | – | – | –
BTW, the norm operator | works