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How do you show the following process is a martingale? My notes say it is a martingale by I can't work it out.

$$ E[e^{\sigma B(t) - \frac{\sigma ^2 t}{2}} | \mathscr{F}(s)] $$

I tried to multiply and divide by $e^{\sigma B(s) - \frac{\sigma ^2 s}{2}}$ but got stuck.

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