A divergence formula for regularization methods with an L2 constraint
  We derive a divergence formula for a group of regularization methods with an L2 constraint. The formula is useful for regularization parameter selection, because it provides an unbiased estimate for the number of degrees of freedom. We begin with deriving the formula for smoothing splines and then extend it to other settings such as penalized splines, ridge regression, and functional linear regression. 
Yixin Fang, Yuanjia Wang, and Xin Huang
stat.OT