The order of the sandpile group is again the determinant of <math>\Delta'</math>, which by the general version of the [[Kirchhoff's theorem|matrix tree theorem]] is the number of oriented [[spanning tree]]s rooted at the sink. | The order of the sandpile group is again the determinant of <math>\Delta'</math>, which by the general version of the [[Kirchhoff's theorem|matrix tree theorem]] is the number of oriented [[spanning tree]]s rooted at the sink. |