Hello
Because v appears as the integration variable as a subscript. Try
Integrate[E^(1/2 (-(v^2/
\!(*SubsuperscriptBox[([Sigma]), (vvvv), (2)]))))/
Sqrt[2 [Pi]] , {v, -Infinity, Infinity}]
For some reason the "code block <>" is not appearing on this community editor. Sorry, cut and paste should work. Adding an image even though this is "bad practice"
