e^-r^2
dx=drcost=costdr-rsintdt
dy=drsint=sintdr+rcostdt
dx^dy=rcost^2dr^dt-rsint^2dt^dr=rdr^dt
re^-r^2dr^dt=-e^-r^2(.5)(2pi)=-pie^-r^2 r=0->∞=-pi(0-1)=π