On the Chebyshev properties of system of eigenfunctions for   Sturm--Liouville problem with singular coefficients
  In the paper we consider singular spectral Sturm--Liouville problem $-(py')'+(q-\lambda r)y=0$, $(U-1)y^{\vee}+i(U+1)y^{\wedge}=0$, where function $p\in L_{\infty}[0,1]$ is uniformly positive, generalized function $q\in W_2^{-1}[0,1]$ is real-valued, generalized weight function $r\in W_2^{-1}[0,1]$ is positive and unitary matrix $U\in\mathbb C^{2\times 2}$ is diagonal. The goal is to prove that well-known (for smooth case) facts about Chebyshev property of eigenfunctions hold in general case. 
A.A.Vladimirov