Crosscap number
= Crosscap number
{title2=$h=2-\chi$}
For a closed connected <nonorientable surface>, the crosscap number is the number of <real projective plane> summands in its <connected sum> classification. It is at least one, and its <Euler characteristic> is $2-h$. The projective plane has one crosscap and the <Klein bottle> has two.