4. A general predictor matrix X
In this section, we consider a general predictor matrix X, with columns in general position. Recall that our proposed covariance test statistic (5) is closely intertwined with the knots λ1 ≥ … ≥ λr in the lasso path, as it was defined in terms of difference between fitted values at successive knots. Moreover, Lemma 1 showed that (provided there are no sign changes in the reduced lasso problem over [λk+1, λk]) this test statistic can be expressed even more explicitly in terms of the values of these knots. As was the case in the last section, this knot form is quite important for our analysis here. Therefore, it is helpful to recall [Efron et al. (2004), Tibshirani (2013)] the precise formulae for the knots in the lasso path. If A denotes the active set and sA denotes the signs of active coefficients at a knot λk,
A=supp(βˆ(λ)),sA=sign(βˆA(λk)),