Bernstein Operators for Extended Chebyshev Systems

Let $U_{n}\subset C^{n}[ a,b] $ be an extended Chebyshev space of dimension $n+1$. Suppose that $f_{0}\in U_{n}$ is strictly positive and $% f_{1}\in U_{n}$ has the property that $f_{1}/f_{0}$ is strictly increasing. We search for conditions ensuring the existence of points $% t_{0},...,t_{n}\in [ a,b] $ and positive coefficients $\alpha_{0},...,\alpha_{n}$ such that for all $f\in C[ a,b]$, the operator $B_{n}:C[ a,b] \to U_{n}$ defined by $% B_{n}f=\sum_{k=0}^{n}f(t_{k}) \alpha_{k}p_{n,k}$ satisfies $% B_{n}f_{0}=f_{0}$ and $B_{n}f_{1}=f_{1}.$ Here it is assumed that $% p_{n,k},k=0,...,n$, is a Bernstein basis, defined by the property that each $% p_{n,k}$ has a zero of order $k$ at $a$ and a zero of order $n-k$ at $b.$